Chapter XII: Section I: MONOCYRTIDA, Haeckel, 1862, Monogr. d. Radiol., pp. 280, 281 (1)

_Definition._--Cyrtoidea monothalamia, with simple, not jointed shell (or cephalis), without transverse constrictions.

_Synopsis of the three Families and six Subfamilies of Monocyrtida._

Family LIX. Tripocalpida. } Mouth open, 1. Archipilida.
}
Three radial apophyses. } Mouth closed, 2. Archiperida.

Family LX. Phænocalpida. } Mouth open, 3. Archiphormida.
}
Numerous radial apophyses. } Mouth closed, 4. Archiphænida.

Family LXI. Cyrtocalpida. } Mouth open, 5. Archicorida.
}
No radial apophyses. } Mouth closed, 6. Archicapsida.

Family LIX. #TRIPOCALPIDA#, n. fam.

_Archipilida et Archiperida_, Haeckel, 1881, Prodromus, pp. 427, 429.

_Definition._--Monocyrtida triradiata. (#Cyrtoidea# with a simple, not jointed shell, representing a simple cephalis, with three radial apophyses.)

{1134}The family Tripocalpida, composed of the Archipilida and Archiperida of my Prodromus, comprises those #Cyrtoidea# in which the lattice-shell is quite simple, without transverse constriction, and bears three radial apophyses. The two subfamilies differ in the shape of the basal mouth, which in the Archipilida is a simple wide opening, in the Archiperida closed by a lattice-plate; the former are here divided into eight, the latter into seven different genera.

Only three species of this family were hitherto described, all three belonging to the Archipilida, viz., (1) _Tripocalpis galea_ (fossil in Barbados, figured by Ehrenberg, 1875, as _Halicalyptra galea_), (2) _Tripodiscium campanula_ (fossil in Sicily, figured by Stöhr, 1880, as _Carpocanium_); and (3) _Tridictyopus elegans_, of which Richard Hertwig gave an excellent description in 1879, with an accurate figure of the central capsule. All the other species of the family (seventy) are new.

The shell in the majority of Tripocalpida is ovate or campanulate, sometimes conical or three-sided pyramidal. Three radial apophyses are constantly distinct, either three lateral wings or three terminal feet; these are usually solid spines, rarely lattice-plates. The top of the shell usually bears an apical horn, rarely two or more horns; often the horn is wanting. The lattice-work of the shell is very various in the different species, and often of a remarkable structure (Pl. 51, figs. 6-8; Pl. 98, figs. 4-8). The cavity of the shell is in all Archipilida, and in a part of the Archiperida (_Peridium_, _Archipera_, _Archibursa_) quite simple. In the other part of the Archiperida, however, constituting the peculiar group of Euscenida (genera 504-507), a vertical columella arises in the centre of the basal plate, and is prolonged upwards in the apical horn; this columella is either simple (_Euscenium_, _Archiscenium_) or branched (_Cladoscenium_, _Pteroscenium_); in the latter case the ascending branches are disposed in triradiate verticils and are inserted on the inner face of the shell (compare Pl. 53, figs. 11-16, and Pl. 98, figs. 1-4).

The phylogenetic origin of the Tripocalpida may be very divergent, and their morphological affinity to the other #Nassellaria# is a very complicated problem. The Euscenida (genera 504-507) may be derived directly from the Plagonida (_Plagoniscus_) or Plectanida (_Plectaniscus_). The Archibursida (genera 508-510) however, manifest a closer affinity to the Tripospyrida, and may be derived from them by reduction of the sagittal ring and constriction. In these two groups of Archiperida the shell may be regarded as a true cephalis. In the Archipilida, however, where the shell has a wide basal opening and the three radial ribs arise originally from the base of the apical horn, the shell itself may correspond to the thorax of the Dicyrtida, and may be derived from these by loss of the cephalis. This is the more probable, as sometimes a small remnant of the reduced cephalis is preserved (Pl. 98, fig. 8). None of these difficult questions can be answered until a much closer morphological knowledge of the Tripocalpida is acquired.

{1135}_Synopsis of the Genera of Tripocalpida._

-------------------------------------------------------------------------
I. Subfamily Archipilida. Basal mouth of the shell a simple wide opening.
--------------------
Shell with three lateral ribs or wings, with or without terminal feet.
-------------------------------------------------------------------------
{ Apex with a horn, 496. _Tripocalpis_.
Three terminal feet. {
{ No apical horn, 497. _Tripodonium_.

Six to nine or more } No apical horn, 498. _Tripterocalpis_.
terminal feet. }

No terminal feet. { Apex with a horn, 499. _Trissopilium_.
{
{ No apical horn, 500. _Archipilium_.
-------------------------------------------------------------------------
Shell without lateral ribs or wings but with three terminal feet.
-------------------------------------------------------------------------
Feet simple or } Apex with a horn, 501. _Tripilidium_.
branched, not latticed. }
} No apical horn, 502. _Tripodiscium_.

Feet latticed. Apex with a horn, 503. _Tridictyopus_.
-------------------------------------------------------------------------
II. Subfamily Archiperida.
Basal mouth of the shell closed by a lattice-plate.
--------------------
Shell in its axis with a free (simple or branched) internal
columella,
prolonged into an apical horn.
-------------------------------------------------------------------------
Shell with three } Columella simple, 504. _Euscenium_.
free feet, without }
lateral lattice-wings. } Columella branched, 505. _Cladoscenium_.

Shell with three } Columella simple, 506. _Archiscenium_.
lateral lattice-wings, }
connecting the three } Columella branched, 507. _Pteroscenium_.
feet with the horn. }
-------------------------------------------------------------------------
Shell with simple cavity, without free columella. Three free basal feet.
-------------------------------------------------------------------------
One apical horn, 508. _Peridium_.

Two or more horns, 509. _Archipera_.

No apical horn, 510. _Archibursa_.

Subfamily 1. ARCHIPILIDA, Haeckel, 1881, Prodromus, p. 427.

_Definition._--#Tripocalpida# with the basal mouth of the shell open (vel Monocyrtida triradiata aperta).

Genus 496. _Tripocalpis_,[113] Haeckel, 1881, Prodromus, p. 428.

_Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) with three lateral ribs prolonged into three terminal feet. Apex with a horn.

The genus _Tripocalpis_ opens the series of Archipilida, or of those Monocyrtida in which the monothalamous shell exhibits a distinct triradial structure, and a simple open mouth, without cortinar lattice. _Tripocalpis_ is probably the most primordial of the Archipilida, and has three lateral ribs, which are prolonged into three free terminal {1136}feet, and connected in the apex of the pyramidal or campanulate shell with the apical horn. It may probably be derived from _Dictyophimus_ by loss of the cephalis. The central capsule is simple, spherical, or ellipsoidal.

1. _Tripocalpis plectaniscus_, n. sp.

Shell three-sided pyramidal, smooth, about as long as broad, with irregular polygonal pores and thin bars. Apical horn stout, three-sided pyramidal, about as long as the shell. From the three edges of its base arise three prominent straight radial ribs, which are prolonged into three large divergent feet, of equal size and the same form as the horn.

_Dimensions._--Shell 0.1 diameter, horn and feet 0.12.

_Habitat._--Tropical Pacific (Philippine Sea), Station 206, depth 2100 fathoms.

2. _Tripocalpis galea_, Haeckel.

_Halicalyptra galea_, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss.
Berlin, p. 74, Taf. ii. fig. 10.

Shell campanulate, rough, about as long as broad, with irregular roundish pores and thick bars. Apical horn stout, three-sided pyramidal, scarcely one-fourth as long as the shell. Beyond its base there arise three prominent, slightly curved lateral ribs, which are prolonged into three pyramidal, slightly divergent stout feet, somewhat shorter than the shell.

_Dimensions._--Shell 0.08 to 0.09 diameter, horn 0.02, feet 0.07.

_Habitat._--Fossil in Barbados.

3. _Tripocalpis tricostata_, n. sp.

Shell ovate campanulate, one and a half times as long as broad, with regular circular pores, disposed in alternate longitudinal rows. Apical horn three-sided pyramidal, stout, half as long as the shell. In the middle of the shell there arise three wing-like lateral ribs, which are prolonged into three broad, slightly curved, divergent terminal feet.

_Dimensions._--Shell 0.12 long, 0.08 broad, horn and feet 0.06 to 0.08.

_Habitat._--Fossil in Barbados.

4. _Tripocalpis triserrata_, n. sp. (Pl. 51, fig. 6).

Shell ovate, nearly twice as long as broad. The three sides of the shell, between the three large ribs, are provided each with nine delicate parallel, longitudinal, slightly curved ribs, alternating with ten longitudinal rows of regular circular pores about as broad as the bars. Apical horn short and broad, three-sided pyramidal; from the three edges of its base there arise three wing-like serrate main ribs, which are prolonged into three short pyramidal terminal feet.

_Dimensions._--Shell 0.12 long, 0.07 broad, horn and feet 0.01.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

{1137}5. _Tripocalpis cortinaris_, n. sp. (Pl. 97, fig. 3).

Shell hemispherical, one and a half times as broad as long, with regular, circular, hexagonally framed pores, Apical horn stout, three-sided pyramidal, with three serrate edges. From the base of the latter arise three strongly dentate and curved lateral ribs, with recurved teeth, and these are prolonged into the serrate convex edge of the three basal feet, which are crescentic and twice as long as the shell.

_Dimensions._--Shell 0.07 long, 0.1 broad; horn 0.06 long, feet 0.14 long.

_Habitat._--Indian Ocean, Cocos Islands (Rabbe), surface.

Genus 497. _Tripodonium_,[114] Haeckel, 1881, Prodromus, p. 428.

_Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) with three lateral ribs prolonged into three terminal feet. Apex without horn.

The genus _Tripodonium_ differs from the preceding _Tripocalpis_, its probable ancestral form, only in the loss of the apical horn (already very small in some forms of the latter). It bears therefore to the latter the same relation that _Sethopilium_ does to _Dictyophimus_.

1. _Tripodonium campanulatum_, n. sp.

Shell campanulate, somewhat broader than high, with regular circular pores, twice as broad as the bars. Three broad lateral wings, lamellar, triangular, are prolonged over the peristome into three short triangular lamellar feet one-fourth as long as the shell.

_Dimensions._--Shell 0.08 long, 0.09 broad; feet 0.02 long.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

2. _Tripodonium ovatum_, n. sp.

Shell ovate, one and one-third times as long as broad, with numerous and small circular pores, scarcely as broad as the bars. Three prominent longitudinal ribs, arising in the lower half of the shell-wall, are prolonged into three divergent conical curved feet, about half as long as the shell.

_Dimensions._--Shell 0.12 long, 0.09 broad; feet 0.06 long.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

Genus 498. _Tripterocalpis_,[115] Haeckel, 1881, Prodromus, p. 428.

_Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) with three lateral wings and a peristomial corona of numerous terminal feet. Shell ovate, with constricted mouth, without apical horn.

{1138}The genus _Tripterocalpis_ is distinguished from the other Archipilida by the remarkable combination of three lateral wings and of numerous (six to nine or more) terminal feet. It may be derived directly from the preceding _Tripocalpis_ by multiplication of the terminal feet. These are sometimes obliquely directed. The central capsule is ellipsoidal or ovate, and fills up the greater part of the shell.

1. _Tripterocalpis phylloptera_, n. sp. (Pl. 51, fig. 1).

Shell slender, ovate, nearly twice as long as broad. Pores circular, of different sizes and at unequal distances. Along the lower half of the shell there arise three broad triangular lamellar wings. Peristome with twelve conical, nearly parallel and vertical feet, about one-sixth as long as the shell.

_Dimensions._--Shell 0.2 long, 0.12 broad; wings 0.1 long, feet 0.03 long.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

2. _Tripterocalpis conoptera_, n. sp. (Pl. 51, fig. 2).

Shell ovate, nearly as broad as long. Pores regular, circular, quincuncial, twice as broad as the bars. In the middle of the shell there arise three conical, smooth, divergent wings, about half as long as the shell. Peristome with six short triangular, oblique, convergent feet.

_Dimensions._--Shell 0.16 long, 0.14 broad; wings 0.09 long; feet 0.02 long.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

3. _Tripterocalpis ogmoptera_, n. sp. (Pl. 51, figs. 3-5).

Shell ovate, nearly as broad as long. Pores small and numerous, circular or roundish, partly confluent, double-contoured (fig. 4). In the middle of the shell there arise three very large conical divergent wings, which are longitudinally striped and longer than half the shell. Peristome with nine short triangular, oblique, convergent feet.

_Dimensions._--Shell 0.18 long, 0.16 broad; wings 0.12 long, feet 0.02 long.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

Genus 499. _Trissopilium_,[116] Haeckel, 1881, Prodromus, p. 427.

_Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) with three lateral ribs or wings. Mouth smooth, without terminal feet. Apex with a horn.

The genus _Trissopilium_ and the following nearly allied _Archipilium_ differ from the other Archipilida in the absence of terminal feet, the peristome being quite simple, and truncate. But there are three lateral wings, arising either from the apex or from the lateral sides of the monothalamous shell. _Trissopilium_ may perhaps be derived from _Lithomelissa_ by reduction of the cephalis.

{1139}1. _Trissopilium tetraplecta_, n. sp.

Shell in the upper half three-sided pyramidal, in the lower half inversely hemispherical, of equal length and breadth. Pores irregular, roundish, in the upper half larger. From the apex there diverge four equal, straight and stout, three-sided pyramidal spines, one of which is vertically ascending (the horn), the three others obliquely descending (the wings). The distal half of the latter is free, whilst the proximal half forms three ribs, enclosed in the shell-wall. Mouth constricted, half as broad as the shell.

_Dimensions._--Shell 0.08 long and broad, horn and wings 0.1 long.

_Habitat._--Central Pacific, Station 268, depth 2900 fathoms.

2. _Trissopilium lithomelissa_, n. sp.

Shell ovate, one and one-third times as long as broad. Pores circular, of different sizes, small and numerous. Apical horn very large, straight, cylindrical, vertical, longer than the shell. Three lateral wings shorter, arising from the middle of the shell, conical, curved, divergent. Mouth truncate, two-thirds as broad as the shell.

_Dimensions._--Shell 0.11 long, 0.08 broad; horn 0.13 long, wings 0.09 long.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

Genus 500. _Archipilium_,[117] Haeckel, 1881, Prodromus, p. 427.

_Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) with three lateral ribs or wings. Mouth truncate, without terminal feet. Apex without horn.

The genus _Archipilium_ differs from the nearly allied _Trissopilium_, its probable ancestral form, in the absence of the apical horn. The three lateral wings of the ovate shell are stout spines. It may also be derived from the similar _Sethopilium_ by reduction and loss of the cephalis.

1. _Archipilium orthopterum_, n. sp. (Pl. 98, fig. 7).

Shell ovate, smooth, nearly cylindrical, scarcely broader than the truncate, widely open mouth. Pores numerous, irregular, roundish, of very different sizes. Three lateral wings arising from the middle zone, twice as long as the shell, straight, divergent, stout, cylindrical, longitudinally striped.

_Dimensions._--Shell 0.08 long, 0.06 broad; wings 0.15 long.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

2. _Archipilium sigmopterum_, n. sp.

Shell ovate, smooth, nearly campanulate, of equal length and breadth, one and one-third times as broad as the truncate, slightly constricted mouth. Pores scarce, irregularly formed and {1140}scattered, of very different sizes. Three lateral wings, stout, prismatic, about as long as the shell widely divergent and S-shaped, curved.

_Dimensions._--Shell 0.08 diameter, wings 0.08 long.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

3. _Archipilium cyrtopterum_, n. sp.

Shell ovate, rough, twice as broad as the constricted mouth. Pores small and numerous, subregular, circular, smaller than the bars. Three lateral wings cylindrical, crescentic, in the upper half divergent, in the lower convergent, one and one-half times as long as the shell.

_Dimensions._--Shell 0.09 long, 0.07 broad; wings 0.14 long.

_Habitat._--Central Pacific, Station 265, depth 2900 fathoms.

Genus 501. _Tripilidium_,[118] Haeckel, 1881, Prodromus, p. 428.

_Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) without lateral ribs, with three simple or branched terminal feet and an apical horn.

The genus _Tripilidium_ differs from _Tripodonium_ in the absence of the three lateral ribs, and may be derived either from this ancestral form, or directly from _Tripospyris_ (by loss of the sagittal ring and constriction); perhaps also from _Lychnocanium_ (by reduction of the cephalis). We distinguish as two subgenera, _Tristylocorys_ (with simple feet) and _Tripodocorys_ (with branched feet).

Subgenus 1. _Tristylocorys_, Haeckel.

_Definition._--Feet simple, not branched.

1. _Tripilidium cortina_, n. sp.

Shell campanulate, rough, about as long as broad. Pores subregular, circular, larger in the middle zone, twice to three times as broad as the bars. Mouth constricted, half as broad as the shell. Apical horn straight and stout, three-sided pyramidal, about as long as the shell. Three feet divergent, slightly S-shaped, curved, somewhat longer the the shell.

_Dimensions._--Shell 0.1 diameter, horn 0.09 long, feet 0.13 long.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

2. _Tripilidium hemisphæricum_, n. sp.

Shell hemispherical, smooth, nearly twice as broad as long. Pores large, with thin bars, irregular, roundish, of very different sizes. Mouth wide open. Apical horn conical, half as long as {1141}the shell. Three feet slender cylindrical, widely divergent, curved, nearly twice as long as the shell.

_Dimensions._--Shell 0.08 long, 0.14 broad; horn 0.05 long, feet 0.15 long.

_Habitat._--South Pacific, Station 298, depth 2225 fathoms.

3. _Tripilidium lychnocanium_, n. sp.

Shell hemispherical, thorny, one and a half times as broad as long. Pores regular, circular, hexagonally framed, as broad as the bars. Mouth slightly constricted. Apical horn stout and short, three-sided pyramidal. Three feet cylindrical, straight, divergent, twice to three times as long as the shell. (Similar to _Lychnocanium favosum_, Pl. 61, fig. 6, perhaps derived from it by reduction of the cephalis.)

_Dimensions._--Shell 0.08 long, 0.12 broad; horn 0.02 long, feet 0.2 long.

_Habitat._--Central Pacific, Station 265, depth 2900 fathoms.

4. _Tripilidium ovatum_, n. sp.

Shell ovate, smooth, one and one half times as long as broad. Pores regular, circular, as broad as the bars. Mouth constricted, half as broad as the shell. Horn short, conical. Three feet conical, curved, convergent, scarcely one-third as long as the shell.

_Dimensions._--Shell 0.15 long, 0.11 broad; horn 0.02 long, feet 0.04 long.

_Habitat._--South Atlantic, Station 332, depth 2200 fathoms.

5. _Tripilidium clavatum_, n. sp.

Shell nearly spherical, tuberculate, a little broader than long. Pores regular, circular, as broad as the bars. Mouth constricted, half as broad as the shell. Apical feet and the three divergent feet of equal size and similar form, about as long as the shell, straight, in the basal half smooth, cylindrical, in the distal half club-shaped, dimpled. (Very similar to _Tripospyris conifera_ and _Tripospyris eucolpos_, Pl. 84, figs. 7 and 4; but the spherical shell exhibits no trace of sagittal ring or constriction, and the simple shell-cavity has a wide open mouth, and is not closed by a basal plate.)

_Dimensions._--Shell 0.1 diameter, horn and feet 0.08 to 0.09 long.

_Habitat._--Central Pacific, Station 268, depth 2900 fathoms.

6. _Tripilidium costatum_, n. sp. (Pl. 98, figs. 8, 8_a_, 8_b_).

Shell ovate, spinulate and costate, nearly twice as broad as long. Surface with fifteen to twenty prominent longitudinal ribs, which are convergent towards each pole and elegantly denticulate. The deep furrows between them are divided by delicate, parallel, transverse ribs into numerous short and broad dimples (thirty to forty in each furrow); each dimple contains a small pore, like a transverse {1142}fissure. The apex bears a short and stout, three-sided pyramidal horn, the hollow base of which is closed by a small cortinar septum with three collar pores (fig. 8_b_). This seems to indicate a rudimentary cephalis and the derivation of this species from _Lychnocanium_ (Pl. 61). Peristome constricted, only one-fourth as broad as the shell, with three short, conical, nearly vertical feet. The central capsule (fig. 8_a_) in the specimen examined was well preserved, hemispherical, not lobate, and filled up the upper half of the shell-cavity (beyond the rudiment of the cortinar septum).

_Dimensions._--Shell 0.2 long, 0.12 broad; horn 0.01 long, feet 0.02 long.

_Habitat._--Tropical Atlantic, Station 348, depth 2450 fathoms.

7. _Tripilidium elongatum_, n. sp.

Shell elongate, smooth, nearly cylindrical, in the upper third conical. Pores small and numerous, regular, circular, half as broad as the bars, disposed in about thirty longitudinal alternating rows. Peristome scarcely constricted. The apical horn and the three parallel feet are of equal size and similar form, straight, conical, one-sixth as long as the shell.

_Dimensions._--Shell 0.12 long, 0.06 broad; horn and feet 0.02 long.

_Habitat._--Fossil in Barbados.

Subgenus 2. _Tripodocorys, Haeckel_, 1881, Prodromus, p. 428.

_Definition._--Feet forked or branched.

8. _Tripilidium dichopodium_, n. sp.

Shell hemispherical, nearly twice as broad as long, smooth. Pores irregular, roundish, twice to four times as broad as the bars. Mouth wide open. Apical horn spindle-shaped, half as long as the shell. Three feet cylindrical, nearly straight and parallel, as long as the shell, forked at the distal end. (Exhibits an external similarity to _Tripospyris semantis_, Pl. 84, fig. 2, but has no trace of sagittal ring and constriction, and the simple shell-cavity has a wide open mouth.)

_Dimensions._--0.06 long, 0.1 broad; horn 0.03 long, feet 0.07 long.

_Habitat._--North Pacific, Station 241, depth 2300 fathoms.

9. _Tripilidium cladopodium_, n. sp.

Shell campanulate, rough, about as long as broad. Pores subregular, circular, as broad as the bars. Mouth slightly constricted. Apical horn and three divergent feet of nearly equal size and form, three-sided prismatic, straight and stout, irregularly dentate and branched, somewhat longer than the shell.

_Dimensions._--Shell 0.08 diameter; horn and feet 0.1 to 0.12 long.

_Habitat._--Central Pacific, Station 265, depth 2900 fathoms.

{1143}Genus 502. _Tripodiscium_,[119] Haeckel, 1881, Prodromus, p. 428.

_Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) without lateral ribs and apical horn, with three simple or branched terminal feet.

The genus _Tripodiscium_ may be derived either from the preceding _Tripilidium_ by loss of the apical horn, or from the nearly related _Tristylospyris_ by loss of the sagittal ring and constriction. Some forms of the latter genus can scarcely be distinguished from similar species of the former.

Subgenus 1. _Tripodiscinus_, Haeckel.

_Definition._--Feet simple, not branched nor forked.

1. _Tripodiscium tristylospyris_, n. sp.

Shell subspherical, with regular, circular pores, twice as broad as the bars. Mouth constricted, half as broad as the shell, with three short spindle-shaped feet, which are nearly parallel and vertical, half as long as the shell. (Similar to _Tristylospyris scaphipes_, Pl. 84, fig. 13, but without sagittal ring and constriction.)

_Dimensions._--Shell 0.08 long, 0.07 broad; feet 0.04 long.

_Habitat._--Central Pacific, Station 265, depth 2900 fathoms.

2. _Tripodiscium ovatum_, n. sp.

Shell ovate, rough, nearly as broad as long. Pores regular circular, double-contoured, twice as broad as the bars. Mouth constricted, one-third as broad as the shell, with three convergent short triangular feet.

_Dimensions._--Shell 0.1 long, 0.09 broad; feet 0.02 long.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

3. _Tripodiscium clavatum_, n. sp.

Shell ovate, smooth, one and a half times as long as broad. Pores irregular roundish, of different sizes. Mouth constricted, two-thirds as broad as the shell, with three divergent, straight club-shaped feet, nearly as long as the shell.

_Dimensions._--Shell 0.12 long, 0.08 broad; feet 0.1 long.

_Habitat._--Tropical Atlantic, Station 348, depth 2450 fathoms.

{1144}4. _Tripodiscium campanula_, Haeckel.

_Carpocanium campanula_, Stöhr, 1880, Palæontogr., vol. xxvi. p. 96, Taf.
iii. fig. 9.

Shell campanulate or subspherical, smooth, about as long as broad. Pores small, regular circular, as broad as the bars. Mouth constricted, about half as broad as the shell, with three short conical descending feet.

_Dimensions._--Shell 0.06 long, 0.05 broad; feet 0.02 long.

_Habitat._--North Pacific, Station 244, depth 2900 fathoms; fossil in Sicily.

Subgenus 2. _Tripodisculus_, Haeckel.

_Definition._--Feet branched or forked.

5. _Tripodiscium furcatum_, n. sp.

Shell ovate, rough, nearly as broad as long. Pores subregular, circular, twice as broad as the bars. Mouth constricted, two-thirds as broad as the shell, with three divergent parallel and vertical feet, which are as long as the shell, and forked at the distal end.

_Dimensions._--Shell 0.09 long, 0.08 broad; feet 0.07 long.

_Habitat._--Tropical Atlantic, Station 338, depth 1990 fathoms.

6. _Tripodiscium ramosum_, n. sp.

Shell ovate, tuberculate, nearly as broad as long. Pores irregular, roundish, of very different sizes. Mouth constricted, half as broad as the shell, with three large, divergent, irregularly branched feet, nearly twice as long as the shell. (Similar to _Tristylospyris ramosa_, Pl. 52, fig. 23, but without sagittal ring and constriction.)

_Dimensions._--Shell 0.07 long, 0.06 broad; feet 0.15 long.

_Habitat._--Central Pacific, Station 274, depth 2750 fathoms.

7. _Tripodiscium sphærocephalum_, n. sp. (Pl. 52, fig. 21).

Shell subspherical, rough, with circular pores of different sizes. Mouth constricted, circular, simple, two-thirds as broad as the shell, with three large divergent feet, which are three-sided prismatic, irregularly branched, and twice as long as the shell.

_Dimensions._--Shell 0.07 long, 0.08 broad; feet 0.12 long.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

{1145}Genus 503. _Tridictyopus_,[120] Haeckel, 1881, Prodromus, p. 428.

_Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) with three latticed terminal feet. Shell ovate or subconical, with an apical horn.

The genus _Tridictyopus_ differs from the nearly allied _Tripilidium_ in the possession of three large fenestrated shovel-shaped feet, forming direct prolongations of the large conical shell. The lattice work exhibits a peculiar elegant structure recurring in the similar, perhaps nearly related, _Cephalospyris_ (Pl. 83, fig. 10; Pl. 96, fig. 28). The central capsule is large, ellipsoidal or ovate, and fills up the greater part of the shell cavity (compare R. Hertwig, 1879, _loc. cit._, p. 75).

1. _Tridictyopus vatillum_, n. sp. (Pl. 51, fig. 8).

Shell campanulate, about as long as broad (excluding horn and feet). Pores irregular, polygonal, in the middle part more regular, triangular. Horn short and stout, three-sided pyramidal. Three feet large, triangular, shovel-shaped, about half as long as the shell.

_Dimensions._--Shell 0.14 long, 0.15 broad.

_Habitat._--South Atlantic, Station 332, depth 2200 fathoms.

2. _Tridictyopus conulus_, n. sp. (Pl. 51, fig. 7).

Shell campanulate conical, about one and a half times as long as broad (excluding horn and feet). Pores mostly triangular, disposed in subregular hexagonal figures, which are separated by bands of smaller irregular polygonal pores. Horn short and stout, three-sided pyramidal. Three feet triangular shovel-shaped, convergent, about one-sixth as long as the shell.

_Dimensions._--Shell 0.3 long, 0.2 broad; horn 0.04 long, feet 0.05 long.

_Habitat._--Equatorial Atlantic, Station 347, depth 2250 fathoms.

3. _Tridictyopus elegans_, R. Hertwig.

_Tridictyopus elegans_, R. Hertwig, 1879, Organismus d. Radiol., p. 75,
Taf. vii. fig. 3, 3_a_, 3_b._

Shell slender, conical, campanulate, about twice as long as broad (excluding horn and feet). Most pores subregular triangular, with some irregular polygonal ones intermingled. The horn in the two observed specimens was broken off, together with the conical apex of the shell. Three feet triangular shovel-shaped, about one-sixth as long as the shell.

_Dimensions._--Shell 0.3 long, 0.15 broad; feet 0.05 long.

_Habitat._--Mediterranean (Messina), R. Hertwig, surface.

{1146}Subfamily 2. ARCHIPERIDA, Haeckel, 1881, p. 429.

_Definition._--#Tripocalpida# with the basal mouth of the shell fenestrated (vel Monocyrtida triradiata clausa).

Genus 504. _Euscenium_,[121] n. gen.

_Definition._--#Archiperida# (vel Monocyrtida triradiata clausa) with a free simple internal columella, prolonged into an apical horn. Three basal feet free, without lateral wings.

The genus _Euscenium_ and the three following nearly allied genera form together the remarkable small group of Euscenida, probably one of the oldest and most primitive of all #Cyrtoidea#, and perhaps the common root of this suborder, arising directly from the #Plectoidea#. The simple skeleton is composed of four radial beams united in one point (the cortinar centrum). Three beams are directed downwards, and form three divergent basal feet; the fourth beam is directed upwards and represents an apical horn; its basal part is enclosed as "columella" in the cavity of the simple shell, the lattice-work of which connects the four spines (compare above, _Plectaniscus_, p. 924). In the genus _Euscenium_, the simplest of all, the columella is simple, and the three feet also simple, without wings.

Subgenus 1. _Euscenarium_, Haeckel.

_Definition._--Three feet simple, not branched.

1. _Euscenium plectaniscus_ (Pl. 98, fig. 1).

Shell three-sided pyramidal, thorny, about as long as broad, with three dentate edges. Framework very loose, with irregular polygonal meshes; in the middle of each of the three triangular sides a horizontal band of denser network. Basal plate with three very large simple collar-holes. The apical horn and the three feet are three-sided pyramidal, about half as long as the columella, with three prominent irregularly dentate edges. The horn is straight, whilst the three feet are hook-shaped, its distal end being curved downwards and inwards.

_Dimensions._--Shell 0.11 long, 0.13 broad.

_Habitat._--Central Pacific, Station 274, depth 2750 fathoms.

2. _Euscenium archicyrtis_, n. sp.

Shell three-sided pyramidal, smooth, nearly as long as broad, with three straight edges connected by irregular loose framework with polygonal meshes. Basal plate with three very large {1147}simple collar-holes. The apical horn and the three feet are three-sided prismatic, smooth, about twice as long as the columella, straight and widely divergent.

_Dimensions._--Shell 0.08 long, 0.1 broad.

_Habitat._--Tropical Pacific, Station 225, depth 4475 fathoms.

3. _Euscenium hemisphæricum_, n. sp.

Shell hemispherical smooth, half as long as broad, with small and numerous subregular circular pores. Basal plate with three large triangular simple collar-holes. Horn slender, three-sided prismatic, twice as long as the columella. Feet very slender, with three denticulate edges, widely divergent and curved like the quadrant of a circle.

_Dimensions._--Shell 0.04 long, 0.06 broad.

_Habitat._--North Pacific, Station 244, depth 2900 fathoms.

4. _Euscenium tricolpium_, n. sp. (Pl. 53, fig. 12).

_Archiscenium tricolpium_, Haeckel, 1881, Prodromus et Atlas, _loc. cit._

Shell cap-shaped, nearly hemispherical, with three vaulted hemispherical bosoms between the three arches ascending from the feet to the horn. Network very irregular, with polygonal meshes, of different form and size. Basal plate with three large primary and some smaller irregular accessory collar-holes. Horn slender, straight, twice as long as the columella. Feet of the same length, slender, widely divergent, slightly curved. The three edges of the horn and the feet are elegantly denticulated.

_Dimensions._--Shell 0.04 long, 0.08 broad.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

5. _Euscenium tripospyris_, n. sp. (Pl. 84, fig. 8).

_Tripospyris euscenium_, Haeckel, 1881, Prodromus et Atlas.

Shell nearly spherical, smooth, with numerous small circular pores. Basal plate also with numerous small pores. Horn and feet stout, three-sided prismatic, straight or slightly curved, about as long as the thin, somewhat curved columella. This species is closely allied to Tripospyris.

_Dimensions._--Shell 0.11 to 0.12 diameter, horn and feet 0.08 to 0.1 long.

_Habitat._--South Pacific, Station 298, depth 2225 fathoms.

Subgenus 2. _Euscenidium_, Haeckel.

_Definition._--Feet forked or branched.

6. _Euscenium furcatum_, n. sp.

Shell campanulate, smooth, with numerous irregular roundish pores. Basal plate with nine pores (three larger central and six smaller distal). Horn club-shaped, half as long as the columella. {1148}Feet twice as long, cylindrical, parallel, vertical, forked at the distal end. (Similar to _Tripospyris semantis_, Pl. 84, fig. 2, but without sagittal feet.)

_Dimensions._--Shell 0.07 long, 0.06 broad; horn 0.04 long, feet 0.1 long.

_Habitat._--Tropical Atlantic, Station 348, depth 2450 fathoms.

7. _Euscenium quadratum_, n. sp.

Shell campanulate, smooth with subregular square pores. Basal plate with three large pores. Horn and feet of the same size and shape, twice as long as the columella, three-sided prismatic, each with four or five verticils of lateral branches. (Similar to the cephalis of _Clathrocorys murrayi_, Pl. 64, fig. 8, but with four free spines without connecting lattice-wings.)

_Dimensions._--Shell 0.06 long, 0.05 broad; horn and feet 0.12 long.

_Habitat._--Central Pacific, Station 272, surface.

8. _Euscenium ramosum_, n. sp.

Shell subspherical, rough, with small subregular circular pores. Basal plate with nine pores (three larger central and six smaller distal pores). Horn and feet of the same size and form, three to four times as long as the columella, three-sided prismatic, irregularly branched.

_Dimensions._--Shell 0.06 diameter, horn and feet 0.15 to 0.2 long.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

Genus 505. _Cladoscenium_,[122] Haeckel, 1881, Prodromus, p. 429.

_Definition._--#Archiperida# (vel Monocyrtida triradiata clausa) with a free branched internal columella, prolonged into an apical horn. Three basal feet free, without lateral wings.

The genus _Cladoscenium_ differs from the preceding _Euscenium_ only in the ramification of the internal columella, which bears one or more verticils of three ascending lateral branches. The three branches of each verticil lie in the same equidistant meridian planes as the three basal feet, and are inserted with their distal ends on the inner surface of the simple lattice-shell. The three feet are also branched, but without vertical lattice-wings. _Cladoscenium_ is perhaps closely allied to _Clathrocorys_.

1. _Cladoscenium fulcratum_, n. sp.

Shell subspherical, smooth, with large irregular roundish pores. Basal plate with three large pores. Columella with a single verticil of three ascending lateral branches. Apical horn as long as the {1149}columella, simple, three-sided prismatic. Feet twice as long, widely divergent, straight, prismatic, with some irregular basal branches.

_Dimensions._--Shell 0.06 diameter, horn 0.05 long, feet 0.1 long.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

2. _Cladoscenium ancoratum_, n. sp. (Pl. 53, fig. 13).

Shell campanulate, smooth, with irregular polygonal pores. Basal plate with six pores (two large cardinal, two smaller jugular and two still smaller cervical pores). Columella with a single verticil of three ascending lateral branches (by mistake not distinct in the figure). Apical horn and the three feet of the same form and size, slender, prismatic, twice to three times as long as the shell, straight, in the distal half with three slender recurved branches, anchor-shaped.

_Dimensions._--Shell 0.05 long, 0.04 broad; horn and feet 0.1 to 0.15 long.

_Habitat._--Central Pacific, Station 274, depth 2750 fathoms.

3. _Cladoscenium gladiatum_, n. sp.

Shell subspherical, with irregular polygonal pores. Basal plate with three large pores. Columella with two verticils of three ascending lateral branches. Apical horn as long as the columella, three-sided prismatic. Feet of the same form, twice as long, in the distal half with two opposite transverse branches, sword-shaped.

_Dimensions._--Shell 0.05 long, 0.06 broad; horn 0.04 long, feet 0.1 long.

_Habitat._--Western Tropical Pacific, Station 225, depth 4475 fathoms.

4. _Cladoscenium pinnatum_, n. sp.

Shell campanulate, with irregular polygonal pores. Basal plate with four pores (two major cardinal and two minor jugular). Columella with two verticils of three ascending lateral branches, and prolonged into a slender prismatic apical horn of twice the length, which bears four such verticils. Feet twice as long as the horn, pinnate, each with four or five pairs of opposite lateral branches, decreasing towards the thin distal end.

_Dimensions._--Shell 0.06 long, 0.05 broad; horn 0.1 long, feet 0.2 long.

_Habitat._--Tropical Atlantic, Station 347, depth 2250 fathoms.

5. _Cladoscenium verticillatum_, n. sp.

Shell campanulate, with subregular square pores. Basal plate with nine pores (three larger central and six smaller distal). Columella with three verticils of three ascending lateral branches. Apical horn and the three basal feet of equal size and similar form, four to six times as long as the columella, three-sided prismatic, each with four to five verticils of three lateral branches, which are {1150}irregularly ramified and tapering towards the distal end. (Similar to _Clathrocorys teuscheri_, Pl. 64, fig. 10, but with four free spines, without connecting lattice-wings.)

_Dimensions._--Shell 0.07 long, 0.06 broad; horn and feet 0.2 to 0.25 long.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

6. _Cladoscenium pectinatum_, n. sp. (Pl. 98, fig. 2).

Shell subspherical, with irregular roundish pores. Basal plate with three large holes. Columella with two verticils of three ascending lateral branches. Apical horn and the three feet of nearly the same size and form, three to four times as long as the columella, straight, three-sided prismatic, each with five to eight verticils of three simple lateral branches, tapering towards the distal end.

_Dimensions._--Shell 0.07 long, 0.08 broad; horn and feet 0.16 to 0.22 long.

_Habitat._--South Atlantic, Station 332, depth 2200 fathoms.

Genus 506. _Archiscenium_,[123] Haeckel, 1881, Prodromus, p. 429.

_Definition._--#Archiperida# (vel Monocyrtida triradiata clausa) with a free simple internal columella, prolonged into an apical horn. Three basal feet connected with the horn by three vertical lateral wings.

The genus _Archiscenium_ and the following nearly allied _Pteroscenium_ are derived from the two preceding genera, from which they differ in the development of three free vertical lateral lattice-wings; these arise from the upper edge of the three basal feet and connect them with the three edges of the apical horn. The basal part of the latter, enclosed in the shell, forms an internal free columella, which in _Archiscenium_ is simple, not branched.

1. _Archiscenium quadrispinum_, n. sp. (Pl. 53, fig. 11).

Shell hemispherical, smooth, with a small number of large irregular polygonal meshes and thin bars. Basal lattice with three very large central meshes and six smaller peripheral meshes (two on the base of each foot). Apical horn and the three feet of equal size and similar form, slender, pyramidal, widely divergent, straight, as long as the shell-diameter. Three lateral wings small, little prominent, each with two or three irregular meshes. Central capsule subspherical.

_Dimensions._--Shell 0.05 long, 0.1 broad.

_Habitat._--Central Pacific, Station 274, depth 2750 fathoms.

2. _Archiscenium clathrocorys_, n. sp.

Shell campanulate, smooth, with numerous irregular square meshes. Basal lattice with three very large meshes. Apical horn and the three feet of equal size and similar form, three-sided prismatic, {1151}widely divergent, straight, twice as long as the shell-diameter. Three lateral wings broad, each with a basal row of few very large irregular roundish meshes, and several distal rows of small square meshes. This species is very similar to _Clathrocorys murrayi_ (Pl. 64, fig. 8), but has no trace of thorax-wall. The latter species may be derived from the former by development of the thoracic lattice, which connects the three free feet.

_Dimensions._--Shell 0.05 long, 0.06 broad.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

3. _Archiscenium tripterygium_, n. sp.

Shell hemispherical, smooth, with numerous irregular roundish meshes of very different sizes. Basal lattice with four large meshes, two major cardinal and two minor cardinal pores. Apical horn and the three feet of similar form and size, slender prismatic, three to four times as long as the shell; the horn straight, the feet curved. Three lateral wings very delicate, with a basal row of few large irregular polygonal meshes, and several distal rows of similar small pores.

_Dimensions._--Shell 0.04 long, 0.05 broad.

_Habitat._--North Pacific, Station 244, depth 2900 fathoms.

4. _Archiscenium cyclopterum_, n. sp. (Pl. 98, fig. 3).

Shell hemispherical, with small irregular roundish pores. Basal lattice with four large meshes, two major cardinal and two minor jugular pores. Apical horn and the three feet of similar form and size, slender prismatic, straight, twice to four times as long as the shell. Three lateral wings nearly semicircular, densely latticed, with the same pores as the shell, and smooth convex margin.

_Dimensions._--Shell 0.05 long, 0.06 broad.

_Habitat._--South Pacific, Station 295, depth 1500 fathoms.

5. _Archiscenium callimitra_, n. sp.

Shell campanulate, with irregular polygonal pores. Basal lattice with nine meshes (three larger central and six smaller distal pores, as in _Callimitra carolotæ_, Pl. 63, fig. 8). Apical horn straight, prismatic, slender, conical, twice as long as the shell. Feet longer and more slender, slightly curved. Three lateral wings very broad, densely fenestrated, with irregular polygonal pores of the same shape as in the similar _Callimitra carolotæ_ (Pl. 63, fig. 1).

_Dimensions._--Shell 0.05 long, 0.04 broad.

_Habitat._--Western Tropical Pacific, Station 225, depth 4475 fathoms.

Genus 507. _Pteroscenium_,[124] Haeckel, 1881, Prodromus, p. 429.

_Definition._--#Archiperida# (vel Monocyrtida triradiata clausa) with a free branched internal columella, prolonged into an apical horn. Three basal feet connected with the horn by three vertical lateral wings.

{1152}The genus _Pteroscenium_ differs from the preceding _Archiscenium_ only in the ramification of the internal columella, and therefore exhibits to it the same relation that _Cladoscenium_ bears to _Euscenium_. The branches of the columella form also here verticils of ascending branches, three in each verticil, and their distal ends are inserted on the inner surface of the lattice-shell.

1. _Pteroscenium arcuatum_, n. sp. (Pl. 98, fig. 4).

Shell campanulate, smooth, nearly conical, somewhat broader than long, with irregular dense network of small circular pores. Columella straight, with two or three triradiate verticils of horizontal lateral branches, which are inserted in the shell-wall. Horn slender pyramidal, as long as the shell. Three divergent feet twice as long as the horn, strongly curved, thorny, three-edged; the distance of their pointed ends twice as broad as the shell. From the outer edge of each foot arise numerous ascending bristles, which are connected together and with similar lateral branches of the horn by curved arches, forming three delicate spiny vertical wings.

_Dimensions._--Shell 0.06 long, 0.08 broad.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

2. _Pteroscenium arcadophorum_, n. sp.

Shell campanulate, smooth, with irregular polygonal pores, very similar to the preceding species but with much broader wings, forming several arcades with three to five rows of irregular meshes. Columella with five or six triradiate verticils of lateral branches. Horn twice as long as the shell, of the same length as the strongly curved spinulate feet.

_Dimensions._--Shell 0.09 long, 0.11 broad.

_Habitat._--Central Pacific, Station 265, depth 2900 fathoms.

3. _Pteroscenium pinnatum_, n. sp. (Pl. 53, figs. 14, 15, 16).

Shell campanulate, smooth, somewhat longer than broad, with irregular delicate network of small roundish pores. Columella straight, with numerous (eight to twelve) triradiate verticils of ascending lateral branches, which are inserted in the shell-wall. The uppermost end of the columella is a conical apical horn. The three divergent feet are smooth, slender, conical, slightly curved, about as long as the shell, pinnate, with six to ten pairs of ascending lateral branches, parallel to those of the columella and inserted on the three vaulted lateral wings. (Fig. 14 exhibits the outside of the shell, fig. 15 the internal structure of the cortina, fig. 16 the basal view.)

_Dimensions._--Shell 0.1 long, 0.12 broad.

_Habitat._--Central Pacific, Station 274, depth 2750 fathoms.

4. _Pteroscenium spinulosum_, n. sp.

Shell campanulate, spinulate, about as long as broad, with small and numerous irregular polygonal pores, very similar to the preceding species, but differing in the more slender form of {1153}the horn and the feet, bearing more numerous lateral branches (twelve to sixteen pairs on each foot). Columella with six to eight triradiate verticils. Horn about as long as the shell.

_Dimensions._--Shell 0.1 long, 0.1 broad.

_Habitat._--Central Pacific, Station 268, depth 2900 fathoms.

5. _Pteroscenium macropodium_, n. sp.

Shell campanulate, spinulate, with irregular polygonal pores; very similar to the two preceding species, but differing in the size and shape of the three feet, which are for the most part solid, triangular, nearly parallel, vertical, very slender, three times as long as the shell, latticed only at the curved base, with few pairs of lateral branches. Columella with four to six triradiate verticils, as long as the stout triangular pyramidal horn.

_Dimensions._--Shell 0.1 long, 0.08 broad.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

6. _Pteroscenium tripocolpum_, n. sp.

Shell campanulate, spinulate, similar to _Euscenium tricolpium_ (Pl. 53, fig. 12) with three vaulted hemispherical bosoms between the three delicate arachnoidal vertical wings. Network loose, very irregular, with polygonal meshes of very different sizes. Columella with three or four triradiate verticils, as long as the slender pyramidal horn. Three feet twice as long, somewhat curved, widely divergent, with three or four pairs of lateral branches. The three edges of the feet and of the horn are elegantly denticulated.

_Dimensions._--Shell 0.06 long, 0.09 broad.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

Genus 508. _Peridium_,[125] Haeckel, 1881, p. 429.

_Definition._--#Archiperida# (vel Monocyrtida triradiata clausa) without internal columella, with an apical horn.

The genus _Peridium_ (or _Archiperidium_, Prodromus, _loc. cit._) and the two following nearly allied genera form together the small group of Archibursida, differing from the four preceding genera (the Euscenida) in the absence of an internal free columella. The three basal feet have therefore no direct connection with the apical horn. _Peridium_, the ancestral form of the Archibursida, may have originated either from _Euscenium_ by reduction and loss of the columella, or directly from _Tripospyris_ by loss of the sagittal ring and the longitudinal constriction.

Subgenus 1. _Peridarium_, Haeckel.

_Definition._--Feet smooth, simple, neither spiny nor branched.

{1154}1. _Peridium lasanum_, n. sp.

Shell subspherical, smooth, with subregular circular pores. Basal plate with three large meshes. Horn apical, vertical, of the same size and form as the three divergent smooth feet, slender pyramidal, twice to three times as long as the shell.

_Dimensions._--Shell-diameter 0.06 to 0.07, length of the three spines 0.15 to 0.2.

_Habitat._--North Pacific, Station 253, depth 3125 fathoms.

2. _Peridium papillatum_, n. sp.

Shell subspherical, papillate, with subregular circular pores. Basal plate with four meshes (two major cardinal and two minor jugular pores). Horn occipital, oblique, three-sided pyramidal, half as long as the shell and as the three widely divergent, subhorizontal, smooth feet.

_Dimensions._--Shell diameter 0.07 to 0.08, length of the horn 0.04, of the feet 0.08.

_Habitat._--Central Pacific, Station 268, depth 2900 fathoms.

3. _Peridium curvipes_, n. sp.

Shell hemispherical, thorny, with irregular roundish pores. Basal plate with six pores (two larger middle cardinal, two smaller anterior jugular and two smaller posterior cervical pores, as in Pl. 53, figs. 13, 18). Horn conical, oblique, arising from the middle of the occipital face, half as long as the shell. Feet slender, conical, curved, smooth, twice as long as the shell.

_Dimensions._--Shell 0.04 long, 0.06 broad; horn 0.02 long; feet 0.08 long.

_Habitat._--Tropical Pacific, Station 206, depth 2100 fathoms.

Subgenus 2. _Archiperidium_, Haeckel.

_Definition._--Feet spiny or branched.

4. _Peridium spinipes_, n. sp. (Pl. 53, fig. 9).

Shell ovate, rough or thorny, with irregular roundish pores. Basal plate with four pores (two larger cardinal and two smaller jugular pores). Horn oblique, occipital, three-sided pyramidal, half as long as the shell, connected with the caudal foot by a prominent crest. Three feet horizontally expanded, prismatic, spiny, about as long as the shell.

_Dimensions._--Shell 0.08 long, 0.06 broad; horn 0.04 long, feet 0.06 to 0.09 long.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

5. _Peridium palmipes_, n. sp. (Pl. 53, fig. 10).

Shell ovate, smooth, with irregular roundish pores. Basal plate with three large pores. Horn oblique, three-sided pyramidal, about as long as the shell, inserted near its occipital base. Three feet prismatic, slender, half as long, ending with a broad dentate plate.

_Dimensions._--Shell 0.06 long, 0.05 broad; horn 0.05 long, feet 0.03 long.

_Habitat._--Western Tropical Pacific, Station 224, depth 1850 fathoms.

{1155}6. _Peridium alatum_, n. sp. (Pl. 97, fig. 7).

Shell subspherical smooth, with regular circular, hexagonally-framed pores. Basal plate with nine pores (three larger central and six smaller distal pores). Horn apical, of equal size and the same form as the three feet, three-sided prismatic, with three broad prominent wings, somewhat longer than the shell.

_Dimensions._--Shell 0.07 long, 0.08 broad; horn and feet 0.1 to 0.12 long.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

7. _Peridium cervinum_, n. sp.

Shell subspherical, spiny, with subregular circular pores. Basal plate with four pores (?). Horn apical, conical, about as long as the shell. Feet three to four times as long, cylindrical curved, with irregular simple or forked branches like the antlers of a deer.

_Dimensions._--Shell diameter 0.1, horn 0.1 long, feet 0.3 to 0.4 long.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

Genus 509. _Archipera_,[126] Haeckel, 1881, Prodromus, p. 429.

_Definition._--#Archiperida# (vel Monocyrtida triradiata clausa) without internal columella, with two or more apical horns.

The genus _Archipera_ differs from the preceding _Peridium_ only in the multiplication of the apical horn, which in the latter is always simple. The number of horns is usually two or three, sometimes four or more. The species bearing three horns may be derived directly from _Triceraspyris_ by loss of the sagittal ring.

1. _Archipera cortiniscus_, n. sp. (Pl. 98, fig. 5).

Shell ovate, thorny, with large roundish pores, which are closed by sieve-plates with numerous small porules. Basal plate with two pairs of simple large pores (two smaller jugular and two larger cardinal, similar to Pl. 98, fig. 10_b_). Two horns divergent, curved, slender, pyramidal; the apical horn longer than the shell, the frontal horn shorter. Three feet divergent, longer than the shell, three-sided prismatic, at the distal end club-shaped, thorny.

_Dimensions._--Shell 0.1 long, 0.08 broad; horns and feet 0.11 to 0.14 long.

_Habitat._--Tropical Pacific, Station 225, depth 4475 fathoms.

2. _Archipera pentacantha_, n. sp.

Shell pear-shaped, smooth, with numerous and small irregular roundish pores. Basal plate with three larger pores. Two horns and three feet of nearly equal size and similar form, three-sided {1156}prismatic, about as long as the shell diameter, somewhat curved. The apical horn stands on the apex, the frontal horn half-way up the shell.

_Dimensions._--Shell 0.08 long, 0.06 broad; horns and feet 0.05 to 0.07 long.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

3. _Archipera bicornis_, n. sp.

Shell ovate, rough, with small regular circular, sparsely scattered pores. Basal plate with four larger central and a peripheral coronal of twelve smaller pores. Two horns near the apex widely divergent, somewhat shorter than the shell, of equal size and similar form. Three feet twice as long, horizontally expanded. All five spines slender, three-sided pyramidal, straight.

_Dimensions._--Shell 0.07 long, 0.06 broad; horns 0.05 long, feet 0.1 long.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

4. _Archipera tricornis_, n. sp.

Shell subspherical, smooth, with subregular circular pores. Basal plate with four large meshes (two major cardinal and two minor jugular pores). On the apex three divergent pyramidal horns of different size, one larger occipital and two smaller frontal horns. Three feet prismatic, about twice as long as the shell.

_Dimensions._--Shell diameter 0.05, length of the feet 0.1.

_Habitat._--North Pacific, Station 253, depth 3125 fathoms.

5. _Archipera quadricornis_, n. sp.

Shell ovate, smooth, with a small number of irregular roundish, widely scattered pores. Basal plate with four crossed meshes of nearly equal size. On the equator there arise four divergent prismatic horns of nearly equal size, three of which correspond to the three feet, whilst the fourth is opposite to the caudal foot. The three feet are also prismatic, as long as the shell, horizontally expanded. (Similar to the upper half of _Lithomelissa decacantha_, Pl. 56, fig. 2, but without thorax.)

_Dimensions._--Shell 0.05 long, 0.04 broad; horns and feet 0.06 long.

_Habitat._--South Pacific, Station 300, depth 1375 fathoms.

6. _Archipera multicornis_, n. sp.

Shell hemispherical, spiny, with irregular polygonal pores. Basal plate with six meshes (?). Between the smaller spines of the surface arise six larger conical divergent horns. Three feet also conical, divergent, about twice as long as the shell and the horns.

_Dimensions._--Shell 0.05 long, 0.07 broad; horns 0.05 long, feet 0.1 long.

_Habitat._--Central Pacific, Station 274, depth 2750 fathoms.

{1157}Genus 510. _Archibursa_,[127] Haeckel, 1881, Prodromus, p. 429.

_Definition._--#Archiperida# (vel Monocyrtida triradiata clausa) without internal columella and without apical horn.

The genus _Archibursa_ has probably arisen from _Peridium_ by loss of the apical horn, and is the only genus of Archiperida which bears no horn. The three basal feet are well-developed. It differs from the similar _Tristylospyris_ by the absence of the sagittal ring and the corresponding longitudinal constriction.

1. _Archibursa tripodiscus_, n. sp. (Pl. 98, fig. 6).

Shell subspherical, smooth, with irregular roundish pores. Basal plate with three large triangular pores of equal size (fig. 6). Three feet widely divergent, straight, three-sided prismatic, about as long as the diameter of the shell.

_Dimensions._--Diameter of the shell 0.07, length of the feet 0.09.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

Family LX. #PHÆNOCALPIDA#, n. fam.

_Archiphormida_ et _Archiphænida_, Haeckel, 1881, Prodromus, pp. 428,
429.

_Definition._--#Monocyrtida# multiradiata. (#Cyrtoidea# with a simple, not jointed shell, resembling a simple cephalis, with numerous radial apophyses (four to nine or more).

The family #Phænocalpida#, composed of the Archiphormida and Archiphænida of my Prodromus, comprises those #Cyrtoidea# in which the lattice-shell is quite without transverse constriction, but bears numerous radial apophyses. The two subfamilies differ in the shape of the basal mouth, which in the Archiphormida is a simple wide opening, but is closed in the Archiphænida by a lattice-plate.

Several Phænocalpida were formerly described by Ehrenberg, viz., _Halicalyptra virginica_ (in 1844) and _Litharachnium arachnodiscus_ (in 1872, called by him _Carpocanium_, though it is quite different from this genus, and closely allied to my _Litharachnium tentorium_, figured in 1862). These forms belong to the Archiphormida (with open mouth). Several fossil Archiphænida (with closed mouth) were figured by Ehrenberg, in 1875, as belonging to _Petalospyris_ (_ocellata_, _carinata_, _flabellum_), though the absence of the sagittal ring and the corresponding longitudinal constriction demonstrates their character as _Phænocalpis_.

{1158}The Phænocalpida are probably an artificial family, composed of three or four (or perhaps more) different groups, which have a quite different origin. The Litharachnida (genera 511-515) and the Haliphormida (genera 516-518) are probably derived from the Sethophormida by loss of the cephalis, the retrograde metamorphosis of which may be observed in all its different stages. The Halicalyptrida (genera 519-521) are perhaps independent of the other Phænocalpida, and may be Archicorida which have produced a corona of teeth around the mouth, without relation to lost radial ribs. The Phænoscenida (_Phænocalpis_, _Phænoscenium_) may be derived from the Euscenida (genera 504-507) by interpolation of interradial feet between the three primary ones, or directly from the #Plectoidea#. The Calpophænida, on the other hand (_Calpophæna_, _Archiphæna_), may have arisen from the Zygospyrida (_Petalospyris_, _Gorgospyris_) by loss of the sagittal ring and the longitudinal constriction. The phylogeny of the Phænocalpida is therefore a very complicated problem, as yet unsolved.

_Synopsis of the Genera of Phænocalpida._

-------------------------------------------------------------------------
I. Subfamily Archiphormida.
Basal mouth of the shell a simple wide opening.
--------------------
Radial ribs enclosed in the wall of the shell, usually prolonged into
free terminal feet.
-------------------------------------------------------------------------
{ Shell simple, with simple
{ network, 511. _Bathropyramis_.
Shell slender, {
pyramidal { Shell simple, with double
(without horn). { network, 512. _Cinclopyramis_.
{
{ Shell double, with an outer
{ mantle, 513. _Peripyramis_.

Shell flatly } Radial ribs simple, 514. _Litharachnium_.
campanulate or }
discoidal (without } Radial ribs branched, 515. _Cladarachnium_.
horn). }

{ Mouth smooth, constricted
Shell ovate, { (no horn), 516. _Cystophormis_.
urceolate or {
campanulate. { Mouth, with { With horn, 517. _Haliphormis_.
{ a corona of {
{ feet. { No horn, 518. _Archiphormis_.
-------------------------------------------------------------------------
No radial ribs in the wall of the shell. Mouth surrounded by a corona of
free terminal feet.
-------------------------------------------------------------------------
{ With horn, 519. _Halicalyptra_.
Shell simple without mantle. {
{ No horn, 520. _Carpocanistrum_.

Shell double, with an external mantle (no horn), 521. _Arachnocalpis_.
-------------------------------------------------------------------------
II. Subfamily Archiphænida. Basal mouth closed by a lattice-plate.
--------------------
In the axis of the shell-cavity a free columella.
-------------------------------------------------------------------------
In the axis of the { Columella simple, 522. _Phænocalpis_.
shell-cavity a {
free columella. { Columella branched, 523. _Phænoscenium_.
-------------------------------------------------------------------------
Shell-cavity simple, without axial columella.
-------------------------------------------------------------------------
Shell-cavity simple, { Apex with a horn, 524. _Calpophæna_.
without axial {
columella. { No apical horn, 525. _Archiphæna_.

{1159}Subfamily 1. ARCHIPHORMIDA, Haeckel, 1881, Prodromus, p. 428.

_Definition._--#Phænocalpida# with the basal mouth of the shell open (vel Monocyrtida multiradiata aperta).

Genus 511. _Bathropyramis_,[128] Haeckel, 1881, Prodromus, p. 428.

_Definition._--#Archiphormida# (vel Monocyrtida multiradiata aperta) with simple, slender pyramidal shell and numerous radial beams (six to nine or more). Network simple, with large, unfenestrated, quadrangular meshes.