Chapter V: Front Matter (5)

Basal ring and frontal ring kidney-shaped, with deep sagittal constriction. Sagittal ring ovate, prolonged at the apex into an oblique horn. From the edges of the angular rings delicate threads arise, which anastomose and form irregular arachnoidal meshes, partly closing the four open lateral gates. Two basal gates simple. From the four nodal points of the basal ring there arise four, short diverging feet, comparable to rudimentary cortinar feet (two lateral from the frontal ring and two sagittal from the primary ring, as basal prolongations).

_Dimensions._--Height of the frontal ring 0.12, breadth 0.18.

_Habitat._--Central Pacific, Station 271, surface.

{980}Genus 420. _Podocoronis_,[39] Haeckel, 1881, Prodromus, p. 445.

_Definition._--#Coronida# with six large gates (four lateral and two basal). Skeleton composed of two complete rings (the sagittal and basal) and one incomplete (frontal) ring. Basal ring provided with typical, regularly disposed, descending feet (cortinar feet).

The genus _Podocoronis_ differs from the two preceding nearly allied genera in the development of typical radial feet on the basal ring, which by their regular disposition and their general appearance among the #Spyroidea# and #Cyrtoidea# obtain great morphological importance. According to the different number and disposition of these basal feet (or "cortinar feet"), the following subgenera may be distinguished (regarded in my Prodromus, 1881, p. 445, as separate genera):--_Dipocoronis_ with two lateral feet, _Tripocoronis_ with three cortinar feet (most important!), _Tetracoronis_ with four crossed feet, _Hexacoronis_ with six radial feet, _Stylocoronis_ with numerous (8 or more feet). These subgenera have important relations to the corresponding genera of Zygospyrida.

Subgenus 1. _Dipocoronis_, Haeckel, 1881, Prodromus, p. 445.

_Definition._--Basal ring with two descending lateral feet (right and left) as prolongations of the frontal ring.

1. _Podocoronis dipodiscus_, n. sp.

Basal ring nearly circular, with two conical, straight, parallel descending feet, which are direct lateral prolongations of the subcircular frontal ring, and half as long as it. The ovate sagittal ring bears at the apex a short conical vertical horn. All rings nearly smooth.

_Dimensions._--Height of the frontal ring 0.12, breadth 0.14.

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

2. _Podocoronis toxarium_, n. sp. (Pl. 83, fig. 7).

Basal ring elliptical, with two descending, slightly curved conical feet, lateral prolongations of the subcircular frontal ring. The ovate sagittal ring bears at the apex a cylindrical vertical horn, which is connected by two descending lateral arches (or galear arches) with the uppermost part of the frontal ring.

_Dimensions._--Height of the frontal ring 0.1, breadth 0.13.

_Habitat._--Central Pacific, Stations 265 to 268, depth 2900 fathoms.

{981}Subgenus 2. _Tripocoronis_, Haeckel, 1881, Prodromus, p. 445.

_Definition._--Basal ring with three descending feet (one caudal and two pectoral).

3. _Podocoronis cortiniscus_, n. sp. (Pl. 97, fig. 2).

Basal ring violin-shaped, with three straight divergent thorny feet, about as long as the ovate sagittal ring. The dorsal part of the latter is straight, vertical, and prolonged downwards into the caudal foot, upwards into a stout, thorny, vertical apical horn. The ventral part is semicircular and gives off the lateral branches, forming the kidney-shaped sagittal ring. All the rings and their apophyses are armed with stout thorns. This and the following tripodal species may be directly derived from _Cortina_.

_Dimensions._--Height of the frontal ring 0.1, breadth 0.15.

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

4. _Podocoronis tripodiscus_, n. sp.

Basal ring kidney-shaped, with three large divergent curved feet, nearly twice as long as the semicircular sagittal ring. The straight dorsal part of the latter is prolonged downwards into the caudal foot, upwards into a large branched apical horn, curved backwards. Differs from the preceding species mainly in the longer curved apophyses and in the larger violin-shaped frontal ring.

_Dimensions._--Height of the frontal ring 0.12, breadth 0.2.

_Habitat._--North Pacific, Station 238, surface. Fossil in Barbados.

Subgenus 3. _Tetracoronis_, Haeckel, 1881, Prodromus, p. 445.

_Definition._--Basal ring with four descending feet (two sagittal and two lateral).

5. _Podocoronis tetrapodiscus_, n. sp.

Basal ring kidney-shaped, with four stout and straight divergent feet, about as long as the sagittal ring; two of them are lateral and arise from the basal corners of the violin-shaped frontal ring (at its junction with the basal ring); two are sagittal, and arise from the basal corners of the semicircular sagittal ring (one posterior caudal and one anterior sternal). The straight dorsal part of the sagittal ring is prolonged upwards into an apical horn. All the rings are thorny.

_Dimensions._--Height of the frontal ring 0.15, breadth 0.25.

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

Subgenus 4. _Hexacoronis_, Haeckel

_Definition._--Basal ring with six radial descending feet (two sagittal, two pectoral, and two tergal feet).

{982}6. _Podocoronis hexapodiscus_, n. sp.

Basal ring violin-shaped, with six conical, vertically descending feet, half as long as the sagittal ring; two of them are sagittal (arising from the two basal corners of the sagittal ring, caudal and sternal feet); the other four are lateral, and prolongations of the forked basal parts of the frontal ring, which is inserted on the basal ring with two forked branches; the anterior correspond to the pectoral, the posterior to the tergal feet of _Hexaspyris_. All the rings are thorny.

_Dimensions._--Height of the frontal ring 0.11, breadth 0.17.

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

Subgenus 5. _Stylocoronis_, Haeckel.

_Definition._--Basal ring with numerous (eight to twelve or more) descending feet.

7. _Podocoronis petalospyris_, n. sp.

Basal ring violin-shaped, with ten to twelve flat lamellar, vertically descending feet, about as long as the ovate sagittal ring. Frontal ring twice as broad as long, elliptical. All rings nearly smooth. Similar to a _Petalospyris_ without lattice-work.

_Dimensions._--Height of the frontal ring 0.1, breadth 0.2.

_Habitat._--Fossil in Barbados.

8. _Podocoronis polypodiscus_, n. sp.

Basal ring elliptical, with fifteen to eighteen conical, curved, descending feet, half as long as the ovate sagittal ring. Frontal ring kidney-shaped, one and a half times as broad as long. All the rings armed with short conical spines.

_Dimensions._--Height of the frontal ring 0.12, breadth 0.18.

_Habitat._--Central Pacific, Station 273, depth 2350 fathoms.

Subfamily 4. TRISSOCYCLIDA, Haeckel, 1881, Prodromus, p. 446.

_Definition._--#Coronida# with eight large gates (four superior lateral and four inferior basal). Skeleton composed of three complete rings, perpendicular to one another; two of which are vertical (the primary or sagittal and the secondary or frontal ring), the third is horizontal (the tertiary or basal ring).

Genus 421. _Tristephanium_,[40] Haeckel, 1881, Prodromus, p. 445.

_Definition._--#Coronida# with eight large, simple gates of different sizes; the four upper or lateral gates larger than the four lower or basal gates. Skeleton composed of three simple complete rings, perpendicular to one another.

{983}The genus _Tristephanium_, and the three following genera derived from it, represent together the interesting subfamily of Trissocyclida (Prodromus, 1881, p. 446). These differ from all other Coronida in the possession of three complete rings, perpendicular to one another. The first of these is the vertical sagittal ring, the second the vertical frontal ring, and the third the horizontal basal ring. Since these three rings lie in the three dimensive planes, they are perpendicular to one another; and between them remain eight large open gates. Originally the four upper or lateral gates (corresponding to those of _Zygostephanus_) are much larger, the four lower or basal gates (corresponding to those of _Semantrum_) much smaller; but afterwards the latter may reach the size of the former, so that the basal ring becomes equatorial (in _Trissocircus_ and _Trissocyclus_). The common ancestral form of the Trissocyclida (_Tristephanium_) may be derived directly either from _Semantrum_ (by complete development of the frontal ring) or from _Eucoronis_ (by complete development of four basal gates).

Subgenus 1. _Triostephus_, Haeckel, 1881, Prodromus, p. 445.

_Definition._--Sagittal and frontal ring of different size and form.

1. _Tristephanium dimensivum,_ n. sp. (Pl. 93, fig. 9).

Sagittal ring ovate, with a large thorny apical horn, smaller than the two other rings. Frontal ring violin-shaped, with a deep sagittal constriction. Basal ring also violin-shaped, in the sagittal axis constricted. Its four gates (two jugular and two cardinal) of nearly equal size. All three rings angular, armed with scattered, stout, thorny spines, which are larger in the basal half.

_Dimensions._--Height of the frontal ring 0.08, breadth 0.12.

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

2. _Tristephanium octopyle,_ n. sp. (Pl. 93, fig. 8).

Sagittal ring ovate, nearly smooth, smaller than the two other rings. Frontal ring kidney-shaped, with a deep basal constriction. Basal ring violin-shaped. The jugular gates about half as large as the cardinal gates. Rods of the rings cylindrical, with few scattered thorns.

_Dimensions._--Height of the frontal ring 0.09, breadth 0.14.

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

3. _Tristephanium hertwigii,_ Haeckel.

_Acanthodesmia hertwigii_, Bütschli, 1882, Zeitschr. f. wiss. Zool., vol.
xxxvi. pp. 499, 539, Taf. xxxii. figs. 9_a_-9_c_.

Sagittal ring ovate or nearly semicircular; its dorsal rod straight, smooth (fig. 9_c_, _a_), its ventral rod (_b_) curved, thorny. Frontal ring much larger, thorny, violin-shaped, with a slight sagittal {984}constriction. Basal ring smaller than the sagittal ring, thorny, kidney-shaped, with four different gates; the two cardinal pores much larger than the two jugular pores. (The four basal pores are often much larger than in the specimen figured by Bütschli. Also the number, form, and size of the spines is very variable.)

_Dimensions._--Height of the frontal ring 0.07 to 0.09, breadth 0.17 to 0.2.

_Habitat._--Fossil in Barbados.

Subgenus 2. _Tristephaniscus_, Haeckel, 1881, Prodromus, p. 445.

_Definition._--Sagittal and frontal ring of equal size and form.

4. _Tristephanium quadricorne_, n. sp. (Pl. 93, fig. 7).

Sagittal and frontal ring of equal size and form, larger than the circular basal ring. From the four corners, in which the latter crosses the two former, arise four strong, divergent spines, branched like a deer's antler. Some smaller spines are scattered on the rings, and a bunch of four spines arises on the apical pole. The four upper gates are triangular, the four lower nearly semicircular, the latter of equal size, half as large as the former.

_Dimensions._--Height of the frontal ring 0.13, breadth 0.14.

_Habitat._--South Atlantic, Station 342, depth 1445 fathoms.

Genus 422. _Tricyclidium_,[41] Haeckel, 1881, Prodromus, p. 446.

_Definition._--#Coronida# with eight large, partly fenestrated gates; the four upper or lateral gates larger than the four lower or basal gates. Skeleton composed of three latticed complete rings, perpendicular to one another.

The genus _Tricyclidium_ differs from the preceding _Tristephanium_, its ancestral form, in the development of loose rudimentary lattice-work along the rings, and therefore bears to it the same relation that _Plectocoronis_ does to _Eucoronis_. It may pass directly over into _Dictyospyris_.

1. _Tricyclidium dictyospyris_, n. sp. (Pl. 93, fig. 13).

Sagittal ring ovate, twice as thick as the two other larger rings, which become very thin, thread-like at the lateral junction, and are both slightly violin-shaped, in the middle sagittal plane constricted; the frontal ring larger than the basal. All three rings bear small scattered spines, which are irregularly branched, and by anastomoses of the thread-like branches form small irregular meshes along the rings. The four basal gates are of equal size.

_Dimensions._--Height of the frontal ring 0.11, breadth 0.15.

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

{985}2. _Tricyclidium semantrum_, n. sp.

Sagittal ring ovate, smaller than the violin-shaped frontal ring and larger than the elliptical basal ring. All three rings of nearly equal thickness, armed with short irregular branches, which are partly connected, and forming small irregular meshes along the rings. The four basal gates are of different sizes; the two anterior (jugular) gates only half as large as the two posterior (cardinal) gates.

_Dimensions._--Height of the frontal ring 0.12, breadth 0.18.

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

Genus 423. _Trissocircus_,[42] Haeckel, 1881, Prodromus, p. 446.

_Definition._--#Coronida# with eight large simple gates of equal size. Skeleton composed of three simple complete rings, perpendicular to one another.

The genus _Trissocircus_, and the following nearly allied _Trissocyclus_, differ from the two preceding ancestral genera in the remarkable growth of the four basal gates, which reach the size of the lateral gates. Therefore both hemispheres of the shell (the upper or apical and the lower or basal) here become equal and the basal ring becomes equatorial. Sometimes even all three rings attain the same size, so that it is difficult or impossible to distinguish them. In this curious case the Coronida exhibit a striking resemblance to some #Sphæroidea#.

Subgenus 1. _Tricircarium_, Haeckel.

_Definition._--Sagittal ring smaller than the two other rings, which are both elliptical.

1. _Trissocircus lentellipsis_, n. sp. (Pl. 93, fig. 10).

All three rings elliptical, of different sizes, smooth, without spines. The sagittal ring is the smallest, but two to three times as thick as the other two rings, which have the larger (transverse) axis common. The smaller (sagittal) axis of the sagittal ring is also the smaller axis of the equatorial ring, whilst the larger (principal) axis of the former is the smaller axis of the frontal ring.

_Dimensions._--Height of the frontal ring 0.07, breadth 0.1.

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

2. _Trissocircus binellipsis_, n. sp. (Pl. 83, fig. 6).

Sagittal ring circular, smaller than the other two rings, which are both equal, elliptical, slightly constricted on the poles of the principal and transverse axes. The axis of the circular sagittal {986}ring is the shorter axis of the frontal and equatorial ring, and about half as long as their longer axis. All three rings smooth, without spines.

_Dimensions._--Height of the frontal ring 0.08, breadth 0.12.

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

3. _Trissocircus octostoma_ (Pl. 93, fig. 11).

Sagittal ring circular, smaller than the other two rings, which are both equal, elliptical, distinctly constricted at the poles of the principal and transverse axes. The axis of the circular sagittal ring is the shorter axis of the frontal and equatorial ring, about one-third shorter than their longer axis. All three rings armed with short, irregularly branched spines.

_Dimensions._--Height of the frontal ring 0.1, breadth 0.12.

_Habitat._--Indian Ocean, Ceylon (Belligemma), Haeckel, surface.

Subgenus 2. _Tricirconium_, Haeckel.

_Definition._--All three rings equal, circular.

4. _Trissocircus octahedrus_, n. sp.

All three rings equal, circular, smooth, their rods cylindrical (in the transverse section circular). From each pole of the three equal axes arises a short conical spine; these six spines correspond to the six corners of a regular octahedron.

_Dimensions._--Diameter of all three rings 0.08, thickness 0.006.

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

5. _Trissocircus globus_, n. sp. (Pl. 82, fig. 13).

All three rings equal, circular, thorny; their rods prismatic (in the transverse section triangular), with three sharp dentated edges and scattered branched thorns. From each pole of the three equal axes (which correspond to the three axes of a regular octahedron) arises a bunch of larger spines.

_Dimensions._--Diameter of all three rings 0.12, thickness 0.01.

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

Genus 424. _Trissocyclus_,[43] Haeckel, 1881, Prodromus, p. 446.

_Definition._--#Coronida# with eight large, partly fenestrated gates of equal size. Skeleton composed of three complete latticed rings, perpendicular to one another.

The genus _Trissocyclus_ differs from its nearly allied ancestral form _Trissocircus_ only in the development of small anastomosing branches along the edges of the rings. The incomplete lattice-work, produced by these anastomoses, partly protects the eight large gates. The three rings are either of equal or of different sizes.

{987}Subgenus 1. _Tricyclarium_, Haeckel.

_Definition._--Sagittal ring circular, smaller than the other two rings, which are both elliptical.

1. _Trissocyclus stauroporus_, n. sp. (Pl. 83, fig. 5).

Sagittal ring circular, smaller than the other two equal elliptical rings. On each of the six corners (or crossing points of every two rings) are developed four small pores, forming a cross around the poles of the three axes. All three rings smooth, without spines.

_Dimensions._--Height of the frontal ring 0.09, breadth 0.13.

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

2. _Trissocyclus triaxonius_, n. sp.

Sagittal ring circular, smaller than the two other equal elliptical rings. From the edges of all three rings are developed small branches, which by irregular ramification and anastomoses form small irregular meshes along the rings. Similar to _Trissocircus octostoma_, but with thicker rings; it may be developed from the latter species by connection of the branches which border the rings.

_Dimensions._--Height of the frontal ring 0.12, breadth 0.15.

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

Subgenus 2. _Tricyclonium_, Haeckel.

_Definition._--All three rings equal, circular.

3. _Trissocyclus sphæridium_, n. sp. (Pl. 93, fig. 12).

All three rings equal, circular, thorny; their rods prismatic, three-sided, with three denticulate edges; the teeth of the lateral edges become branched, and form by anastomoses of the branches small irregular meshes, which partly protect the eight large open gates.

_Dimensions._--Diameter of the three rings 0.16, thickness 0.012.

_Habitat._--Indian Ocean, Belligemma, Ceylon (Haeckel), surface

Family LI. #TYMPANIDA#, Haeckel.

_Parastephida_, Haeckel, 1881, Prodromus, p. 446.

_Definition._--#Stephoidea# with two parallel horizontal rings, an upper mitral and a lower basal ring; both connected by complete or incomplete vertical rings, or by parallel vertical columellæ.

{988}The family #Tympanida# (or Parastephida) differs from the other #Stephoidea# in the development of two horizontal rings, which lie in horizontal planes and arise from the upper and the lower poles of the primary sagittal ring. Therefore the same formation, which we found in the Semantida on the basal pole only of the main axis, here takes place also on its apical pole. On both poles arise at each side from the sagittal ring two corresponding pairs of curved branches: the lateral ends of the opposite branches (dorsal and ventral branch) become united in the frontal plane and so form two complete horizontal rings. The upper ring, composed of the superior or mitral branches, may be called the mitral ring, the lower ring, formed by the inferior or basal branches, the basal ring. Both rings are commonly of nearly equal size, more rarely of different sizes. Their connection is effected either by the primary (sagittal) ring only, or also by secondary vertical rings, a frontal ring in the lateral plane, or some diagonal meridian rings between the latter and the former. These vertical rings, which connect both horizontal rings in the form of "columellæ," may be either complete or incomplete; in the latter case their apical and basal parts are lost, the dorsal and ventral parts only being preserved.

The Tympanida develop a great variety of different forms, greater than in the three other families of #Stephoidea#, so that we may distinguish here not less than sixteen genera and seventy-two species. Hitherto only three species were known: _Prismatium tripleurum_, figured in 1862 in my Monograph, _Tympanidium barbadense_, figured by Bury in the "fossil Polycystins of Barbados," and _Tympaniscus fibula_, described in 1875 by Ehrenberg as a _Ceratospyris fibula_. The number of species in this large and richly developed group will be considerably augmented by further researches. A great many of them exhibit remarkable relations to different NASSELLARIA. For better survey of the whole family we distinguish here four subfamilies, the first of which (Protympanida) is probably the common ancestral group of the other three. They may have been derived originally either from the Semantida or directly from the Stephanida.

The first subfamily, Protympanida, exhibits a complete primary or vertical sagittal ring, bearing on the upper part a horizontal mitral ring, on the lower part a horizontal basal ring. Therefore the gates of these two parallel rings become bisected by the sagittal ring, each divided into a right and a left half. The sagittal ring itself becomes divided by the two horizontal rings into four parts or rods; two opposite of these are more or less vertical; the dorsal (posterior) and the ventral (anterior) rod; the two other rods are more or less horizontal; the mitral (upper) and the basal (lower) rod.

In _Protympanium_ (Pl. 93, fig. 14), the simplest and most primitive form of Tympanida, the connection of the two horizontal rings is effected only by the dorsal and ventral rods of the sagittal ring forming two parallel more or less vertical "columellæ." In all other Protympanida accessory columellæ are developed between the latter. In the greater number we find two secondary columellæ in the lateral plane, being the lateral {989}segments of an incomplete frontal ring (_Acrocubus_, Pl. 93, figs. 15-17). Between the latter and the former there are sometimes developed four tertiary columellæ, as segments of two crossed incomplete diagonal rings (_Tympanidium_, Pl. 94, figs. 1, 18). At other times we find six columellæ; here probably the frontal ring is cleft into halves on each side (_Tympaniscus_, Pl. 94, figs. 4-7). Some genera are distinguished by the development of an equatorial ring, or a third horizontal ring (between the upper mitral and the lower basal ring); this equatorial ring is either complete (_Microcubus_ Pl. 94, figs. 8-10) or incomplete, developed only laterally (_Octotympanum_, Pl. 94, figs. 2, 3). The genus _Toxarium_ (Pl. 93, figs. 18-20) is distinguished by the development of accessory pairs of bows in the frontal plane, upper or galear bows and lower or thoracal bows, remarkable as beginnings of the accessory joints, which we distinguish in many #Spyroidea# and #Cyrtoidea# as "galea and thorax."

The second subfamily of Tympanida are the Paratympanida, in which the two horizontal rings become closed by lattice-work. The numerous columellæ connecting the two rings are here either divergent (_Paratympanum_, with two unequal rings, Pl. 94, fig. 14) or parallel and vertical (_Lithotympanum_, with two equal rings, Pl. 83, fig. 1). The shell of the latter assumes the form of a drum.

A third small subfamily, Dystympanida, is represented by a single genus only, _Dystympanium_ (Pl. 94, figs. 15, 16). Here only the upper or mitral ring is closed by lattice-work, whilst the lower or basal ring exhibits quite a simple opening, a large "basal gate" which has probably arisen by loss of the basilar rod of the sagittal ring. These curious forms exhibit a remarkable resemblance to some Monocyrtida, and also to the _Dictyocha_ (PHÆODARIA).

The fourth subfamily, the Eutympanida, are distinguished by the simple wide aperture of the two horizontal rings; the upper enclosing a simple "mitral gate," the lower a simple "basal gate." Here probably the two horizontal parts of the sagittal ring (the upper mitral rod and the lower basilar rod) are lost by reduction; its two vertical parts (the dorsal and ventral rods) only remaining and forming the two parallel columellæ, which connect the two horizontal rings (_Parastephanus_, Pl. 93, fig. 21). In this group also secondary columellæ are commonly developed between the two rings, alternating with the two primary columellæ. _Pseudocubus_ (Pl. 94, figs. 11, 12), resembling an obelisk, exhibits four divergent columellæ; the two sagittal of these are probably the vertical parts of the primary ring, whilst the two lateral are the halves of an incomplete frontal ring. In _Lithocubus_ (Pl. 82, fig. 12) the four columellæ are parallel and vertical, the two rings being equal; the shell here assumes the form of a geometrical cube. _Prismatium_ is distinguished by three parallel columellæ; two of these are probably parts of a frontal ring, whilst the third is the remains of a reduced sagittal ring. In _Circotympanum_ (Pl. 94, fig. 17) and in _Lithotympanum_ (Pl. 82, fig. 11) the number of the columellæ is augmented, there being six to eight or more; in the former they are divergent {990}(the two rings being of different sizes), in the latter parallel (the two rings being equal); the fundamental form of the former is a truncated pyramid, of the latter a prism.

Comparing these differences between the four subfamilies of Tympanida, we may suppose that the common ancestral group are the Protympanida, derived from the Semantida by the formation of a mitral ring. If the two horizontal rings become closed by lattice-work, the Paratympanida arise. When the mitral ring only becomes latticed, the basal ring being simple, the Dystympanida originate. Finally, the fourth subfamily, the Eutympanida, may be derived from the Protympanida by the loss of the two horizontal parts of the sagittal ring.

The two horizontal rings (upper mitral and lower basal) and the two vertical rings (primary sagittal and secondary frontal), which thus compose the loose framework of the Tympanida, are rarely smooth, commonly armed with spines or branches, similar to those of the Coronida. The rods or bars are either roundish (with circular transverse section) or angular (commonly with triangular transverse section). The paired branches of the two vertical rings often preserve the characteristic arrangement inherited from the Semantida and Stephanida. The two horizontal rings sometimes exhibit typical apophyses, which recur in the #Spyroidea# and #Cyrtoidea#. The mitral ring may possess one odd apical horn and two paired frontal horns (Pl. 93, figs. 16, 17, 19, 23). The basal ring sometimes bears the three typical feet of _Cortina_ (Pl. 93, figs. 16, 22), one odd caudal foot and two paired pectoral feet. Sometimes a fourth (sternal) foot is developed (Pl. 94, fig. 4), at other times only two lateral feet (Pl. 94, figs. 5, 6).

_Synopsis of the Genera of Tympanida._

-------------------------------------------------------------------------
I. Subfamily Protympanida.
Two horizontal rings (upper mitral ring and lower basal ring)
bisected by the complete sagittal ring.
-------------------------------------------------------------------------
Two horizontal rings {One complete sagittal
connected by two { ring (no frontal
columellæ. { ring), 425. _Protympanium_.

} {No galear and
Two horizontal} { thoracal bows, 426. _Acrocubus_.
rings }No equatorial {
connected } ring. {Galear and
by four } { thoracal bows, 427. _Toxarium_.
columellæ }
(the halves of} {Equatorial ring
the sagittal }An equatorial { complete, 428. _Microcubus_.
and the } ring. {
frontal ring).} {Equatorial ring
} { incomplete, 429. _Octotympanum_.

{Six columellæ
Two horizontal rings { (three meridional
connected by six or eight { rings), 430. _Tympaniscus_.
columellæ (three or four {
vertical rings). {Eight columellæ
{ (four meridional
{ rings), 431. _Tympanidium_.
-------------------------------------------------------------------------
II. Subfamily Paratympanida. Two horizontal rings fenestrated.
-------------------------------------------------------------------------
{Two rings unequal
Two horizontal rings closed { (columellæ
by a lattice-plate, { divergent), 432. _Paratympanum_.
connected by numerous {
columellæ. {Two rings equal
{ (columellæ
{ parallel), 433. _Lithotympanum_.

III. Subfamily Dystympanida. Mitral ring fenestrated, basal ring simple.
------------------------------------------------------------------------
Two horizontal rings } Mitral ring closed by a
connected by numerous } lattice-plate, basal
(six to eight or more } ring simple, 434. _Dystympanium_.
columellæ). }

IV. Subfamily Eutympanida. Two horizontal rings (upper mitral ring and
lower basal ring) simple. Apical and basal parts of the sagittal ring
lost.
-----
{Two columellæ (parts of
Two horizontal rings { the sagittal ring), 435. _Parastephanus_.
connected by two or {
three columellæ (four {Three columellæ (parts of
or five open gates). { the half sagittal and
{ the frontal ring), 436. _Prismatium_.

{ {Two rings
{ { unequal, 437. _Pseudocubus_.
{Four {
Two horizontal rings { columellæ {Two rings
connected by four or { { equal, 438. _Lithocubus_.
more columellæ (six or {
eight or more open { {Two rings
gates). {Six to eight { unequal, 439. _Circotympanum_.
{ or more {
{ columellæ. {Two rings
{ { equal, 440. _Eutympanium_.

Subfamily 1. PROTYMPANIDA, Haeckel.

_Definition._--#Tympanida# with two bisected horizontal rings, connected by the complete vertical sagittal ring. (The upper mitral ring and the lower basal ring become divided by the complete primary or sagittal ring into two horizontal symmetrical gates, two mitral gates on the apical and two basal gates on the basal pole.)

Genus 425. _Protympanium_,[44] Haeckel, 1881, Prodromus, p. 447 (_sensu emend._).

_Definition._--#Tympanida# with two bisected horizontal rings, connected by two vertical columellæ.

The genus _Protympanium_, is the most simple form of all Tympanida, and probably the common ancestral form of this family. It arises from _Lithocircus_ by the development of two horizontal rings, perpendicular to the primary sagittal ring. The upper or mitral ring arises by lateral union of two pairs of superior branches, the lower or basal ring by union of two pairs of basal branches of the primary vertical ring.

1. _Protympanium primordiale_, n. sp.

Mitral and apical ring of equal size, elliptical, smooth, somewhat smaller than the connecting thorny sagittal ring.

_Dimensions._--Diameter of the two horizontal rings 0.12, of the sagittal ring 0.15.

_Habitat._--Central Pacific, Stations 263 to 274, at various depths.

{992}2. _Protympanium amphipodium_, n. sp. (Pl. 93, fig. 14).

Mitral and apical rings of equal size, thin, elliptical, smooth, larger than the broad connecting sagittal ring. The latter bears on the apical and on the basal pole two divergent straight spines (lying in the frontal plane), the two apical (or horns) somewhat smaller and not so spiny as the two basal spines (or feet). In the frontal plane there arise from the two horizontal rings on each side two small opposite spines (remnants or beginnings of the lateral frontal ring?).

_Dimensions._--Diameter of the two horizontal rings 0.11, of the sagittal ring 0.07.

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

3. _Protympanium trissocircus_, n. sp.

Mitral ring smaller than the basal ring; both rings circular, rough. Sagittal ring in size between the two former. All three rings covered with very small thorns.

_Dimensions._--Diameter of the mitral ring 0.08, of the sagittal ring 0.1, of the basal ring 0.12.

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

Genus 426. _Acrocubus_,[45] Haeckel, 1881, Prodromus, p. 447.

_Definition._--#Tympanida# with two bisected horizontal rings, connected by four vertical columellæ. No equatorial ring.

The genus _Acrocubus_ is the most primitive of those remarkable Tympanida, the skeleton of which represents more or less exactly the form of a geometrical cube. A simple complete sagittal ring bears on the superior and on the inferior part two opposite pairs of lateral branches; by union of the convergent branches of each side there arise two horizontal parallel quadrangular rings, which are bisected by the sagittal ring. Therefore the superior ring encloses two triangular mitral gates, the inferior two triangular basal gates. The lateral corners of the two gates of each side are connected by a vertical rod or columella. These two parallel columellæ are the remaining middle parts of the secondary or frontal ring, which is incomplete on the superior and on the inferior face of the body. If the sagittal ring also become incomplete, by the loss of the superior and inferior part (the anterior and posterior only remaining), then _Acrocubus_ is transformed into _Lithocubus_.

Subgenus 1. _Apocubus_, Haeckel.

_Definition._--Basal ring without descending feet.

{993}1. _Acrocubus octopylus_, n. sp. (Pl. 82, fig. 9).

Mitral ring somewhat smaller than the basal ring; both rings rhombic, with curved outlines. Sagittal ring elliptical, with six pairs of nodulate protuberances. Four columellæ curved. Nodal points without radial spines.

_Dimensions._--Height of the frontal ring 0.12, breadth 0.18.

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

2. _Acrocubus tesseralis_, n. sp.

Mitral ring of the same size as the basal ring; both rings square, smooth, with straight outlines. Sagittal ring also square, smooth. Four columellæ straight. Nodal points without radial spines. The shell has the form of a regular geometrical cube, the edges of which are represented by the rings.

_Dimensions._--Height of the frontal ring 0.1, breadth 0.11.

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

Subgenus 2. _Dipocubus_, Haeckel.

_Definition._--Basal ring with two descending feet.

3. _Acrocubus brachiatus_, n. sp.

Mitral ring smaller than the basal, both rings square, with thick straight rods. Sagittal ring tuberculate, square, equatorial part thinner. Four columellæ curved, the two lateral ones prolonged downwards into two vertical parallel straight feet of half their length.

_Dimensions._--Diameter of the cube 0.12; length of the feet 0.06.

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

4. _Acrocubus amphistylus_, n. sp.

Mitral and basal rings of nearly equal size, square, with thick curved rods. Sagittal ring elliptical, in the equatorial part thinner. Frontal ring tuberculate. Four columellæ curved, the two lateral ones prolonged downwards into two vertical, tuberculate feet of half their length.

_Dimensions._--Diameter of the cube 0.13; length of the feet 0.07.

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

5. _Acrocubus arcuatus_, n. sp. (Pl. 93, fig. 15).

Mitral and basal rings, as well as the sagittal ring, of the same form as in the preceding nearly allied species. The two lateral columellæ are prolonged not only downwards into two short {994}vertical feet, but also upwards into two curved bows, which are united in a vertical ascending horn, arising from the apical pole of the sagittal ring.

_Dimensions._--Diameter of the cube 0.14; total height 0.21.

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

Subgenus 3. _Tripocubus_, Haeckel.

_Definition._--Basal ring with three descending feet (one odd caudal and two paired lateral).

6. _Acrocubus lasanum_, n. sp.

Mitral ring smaller than the basal, both rings square, with curved rods. Sagittal ring ovate, prolonged upwards into an occipital simple horn, downwards into a caudal foot. Two lateral columellæ prolonged downwards into two pectoral feet. All three feet conical, about as long as the height of the cube.

_Dimensions._--Diameter of the cube 0.08; length of the feet 0.07.

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

7. _Acrocubus cortina_, n. sp. (Pl. 93, fig. 16).

Mitral ring smaller than the basal, both rings elliptical, with curved rods. Sagittal ring ovate, prolonged upwards into an occipital horn, downwards into a caudal foot, as in the preceding species. As in the latter, also, the two lateral columellæ are prolonged downwards into two pectoral feet, but also upwards into two convergent horns, which are united with the occipital horn (commencing the formation of a galea). Rings and feet somewhat thorny.

_Dimensions._--Diameter of the cube 0.16; total height 0.2.

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

Subgenus 4. _Tetracubus_, Haeckel.

_Definition._--Basal ring with four descending feet (two sagittal and two lateral).

8. _Acrocubus tetrapodius_, n. sp.

Mitral and basal rings of the same size, square, with straight rods, sagittal ring also square. The four vertical columellæ are nearly parallel and straight, prolonged upwards into four short conical horns, downwards into four longer conical feet. (The shell has nearly the form of a geometrical cube, the eight corners of which are prolonged into eight vertical conical spines, four smaller ascending and four larger descending.)

_Dimensions._--Diameter of the cube 0.09; length of the feet 0.06.

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

{995}9. _Acrocubus amphithectus_, n. sp. (Pl. 93, fig. 17).

Mitral ring smaller than the basal, both rings elliptical, with curved rods. Four columellæ thorny, also curved, divergent downwards, prolonged upwards into four thorny convergent horns, downwards into four larger branched divergent feet. The two sagittal horns and feet (anterior and posterior) are smaller than the two lateral (right and left); therefore the fundamental form of this species is distinctly amphithect (as in the _Ctenophora_).

_Dimensions._--Breadth of the cube 0.12; total height 0.2.

_Habitat._--South Pacific, Station 289, depth 2550 fathoms.

Genus 427. _Toxarium_,[46] n. gen.

_Definition._--#Tympanida# with two bisected horizontal rings, connected by four vertical columellæ. In the frontal plane two galear bows project upwards, and two thoracal bows downwards.

The genus _Toxarium_ differs from its ancestral form _Acrocubus_ in the development of two pairs of peculiar bows, placed in the lateral or frontal plane; the upper pair projects above the mitral gates, and may be called the "galear bows," the lower pair projects below the basal gates of the cubical shell, and may be called the "thoracal bows." Both pairs are produced originally by the development of two forked vertical spines, arising in the frontal plane on both poles of the principal axis from the sagittal ring (compare _Protympanium amphipodium_, Pl. 93, fig. 14). The apical fork-branches become connected with the lateral corners of the mitral gates, the basal branches with those of the basal gates. By development of lattice-work between the bows and the rings some #Spyroidea# (_Amphispyris_, &c.) may have arisen. The important genus _Toxarium_ may be divided into three subgenera. In _Toxellium_ all the bows are simple, in _Toxonium_, all are forked; in _Toxidium_, the galear bows are simple, the thoracal bows forked.

Subgenus 1. _Toxellium_, Haeckel.

_Definition._--Galear and thoracal bows simple.

1. _Toxarium circospyris_, n. sp. (Pl. 88, fig. 1).

Galear and thoracal bows simple, of about the same size as the frontal bows between them; all bows armed with scattered simple conical spines. Sagittal constriction deep, half as long as the whole shell. Lateral outlines of the shell nearly parallel.

_Dimensions._--Length of the shell 0.16, breadth 0.13.

_Habitat._--Central Pacific, Stations 265 to 274, depth 2350 to 2925 fathoms.

{996}2. _Toxarium subcirculare_, n. sp.

Galear and thoracal bows simple, armed with scattered simple spines of about equal size, smaller than the smooth frontal bows between them. Sagittal constriction very flat. Therefore the frontal perimeter of the shell is nearly circular.

_Dimensions._--Length of the shell 0.15, breadth 0.14.

_Habitat._--North Atlantic (Antilles), Weber, surface.

3. _Toxarium thorax_, n. sp. (Pl. 93, fig. 18).

Galear bows simple, smaller than the frontal bows, and these again smaller than the simple thoracal bows; therefore the shell increases in breadth towards the base, and resembles the skeleton of the thorax of one of the higher vertebrates (the bow-pairs corresponding to rib-pairs, the ventral part of the sagittal ring to the sternum, the dorsal part to the vertebral column). Sagittal constriction very deep. All bows are armed with scattered, irregularly branched spines.

_Dimensions._--Length of the shell 0.17, breadth 0.2.

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

4. _Toxarium constrictum_, n. sp.

Galear and thoracal bows simple, of nearly equal size, larger than the frontal bows between them. Therefore the shell is constricted in the equatorial plane. Sagittal constriction also deep. All bows armed with scattered, irregularly branched spines.

_Dimensions._--Length of the shell 0.14, breadth 0.12.

_Habitat._--North Pacific, Station 236, surface.

Subgenus 2. _Toxidium_, Haeckel.

_Definition._--Galear bows simple, thoracal bows forked.

5. _Toxarium cordatum_, n. sp. (Pl. 93, fig. 19).

Galear bows simple, thoracal bows forked, both somewhat smaller than frontal bows; all bows armed with scattered simple conical spines, mostly developed at the thoracal bows. Both galear bows are united in a central vertical columella, which arises from the apex of the sagittal ring, and is prolonged into an apical horn. Therefore the sagittal constriction exists only in the basal half of the inversely cordate shell.

_Dimensions._--Length of the shell 0.18, breadth 0.16.

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

{997}6. _Toxarium furcatum_, n. sp.

Galear bows simple, thoracal bows forked, both somewhat broader than the frontal bows. Shell therefore slightly constricted in the equatorial plane. Sagittal constriction rather deep, equal in the apical and basal parts. All bows armed with scattered spines, which are partly simple, partly forked.

_Dimensions._--Length of the shell 0.14, breadth 0.16.

_Habitat._--Central Pacific, Station 270, depth 2925 fathoms.

7. _Toxarium pedatum_, n. sp.

Galear bows simple, smaller than the frontal bows, which are again smaller than the thoracal bows. Therefore the breadth of the shell increases towards the base. Sagittal constriction flat in the apical part, deep in the basal part. All bows armed with small irregularly scattered spines; at the base six larger divergent feet (two lateral, two anterior, and two posterior).

_Dimensions._--Length of the shell 0.17, breadth 0.15.

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

Subgenus 3. _Toxonium_, Haeckel.

_Definition._--Galear and thoracal bows forked.

8. _Toxarium costatum_, n. sp.

Galear and thoracal bows forked, smaller than the frontal bows, all bows thorny, without larger spines. Sagittal constriction on both poles deep. Lateral outlines of the shell nearly parallel.

_Dimensions._--Length of the shell 0.15, breadth 0.12.

_Habitat._--Indian Ocean, Socotra (Haeckel), surface.

9. _Toxarium bifurcum_, n. sp. (Pl. 93, fig. 20).

Galear and thoracal bows forked, the former smaller than the frontal bows, the latter larger. Sagittal constriction rather flat on the apical pole, which bears a vertical forked horn, very deep on the basal pole of the primary ring. All bows armed with numerous spines, which are partly simple, partly forked, and mainly developed at the basal part of the thoracal bows.

_Dimensions._--Length of the shell 0.2, breadth 0.2.

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

Genus 428. _Microcubus_,[47] Haeckel, 1881, Prodromus, p. 447.

_Definition._--#Tympanida# with two bisected horizontal rings, connected by four vertical columellæ, which are bisected again by an equatorial ring.

{998}The genus _Microcubus_ is derived from its ancestral form _Acrocubus_ by development of a third horizontal ring, which lies in the equatorial plane, parallel to the upper mitral and the lower basal ring. Whilst these latter are both bisected by the complete vertical sagittal ring, the equatorial ring is complete, and connected at the poles of the sagittal axis with the principal ring, and at the poles of the transverse axis with the frontal ring. Therefore the shell exhibits not eight large gates as in _Acrocubus_, but twelve, viz., four horizontal gates (two superior mitral and two inferior basal), four upper vertical gates (between the mitral and the equatorial ring), and four lower vertical gates (between the basal and the equatorial ring).

1. _Microcubus dodecastoma_, n. sp. (Pl. 94, fig. 9).

All twelve gates simple, irregularly ovate or pentagonal. Frontal ring on the sides concave, constricted by the equatorial ring. All rings curved, armed with short irregular thorns. No larger basal spines.

_Dimensions._--Length of the shell 0.13, breadth 0.15.

_Habitat._--Central Pacific, Stations 271 to 274, depth 2350 to 2750 fathoms.

2. _Microcubus pentacircus_, n. sp.

All twelve gates simple. Frontal ring on the sides convex, not constricted by the equatorial ring. All rings tuberculate, with irregular roundish knots. No larger basal spines. Similar to the preceding species; but the five rings are much thicker and tuberculate (not thorny); the twelve gates are therefore relatively smaller.

_Dimensions._--Length of the shell 0.1, breadth 0.12.

_Habitat._--Fossil in Barbados.

3. _Microcubus zonarius_, n. sp. (Pl. 94, fig. 8).

All twelve gates simple, ovate. Frontal ring on the sides convex, not constricted by the equatorial ring, nearly twice as broad as long. All rings thorny, very thin, except only the broad angular sagittal ring. No larger basal spines.

_Dimensions._--Length of the shell 0.08, breadth 0.15.

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

4. _Microcubus quadrupes_, n. sp.

All twelve gates simple, square. Frontal ring square, not constricted by the equatorial ring. All rings smooth, quadrangular. Basal ring with four short vertical conical feet, two of which (right and left) are inferior prolongations of the frontal ring, and arise from the lateral corners of the {999}basal gates; two others are prolongations of the sagittal ring (sternal and caudal), and arise from the medial corners of the basal gates. (The shell is like a small cube with four feet.)

_Dimensions._--Length of the shell 0.13, breadth 0.11.

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

5. _Microcubus cornutus_, n. sp.

Eight gates (two mitral, two basal, and four superior) simple; four inferior gates fenestrated by arachnoidal irregular threads. Frontal ring square, constricted on the sides by the equatorial ring. Basal ring with four short conical vertical feet, as in the preceding species. Sagittal ring with a vertical horn on the apex. All rings with small scattered thorns.

_Dimensions._--Length of the shell 0.11, breadth 0.13.

_Habitat._--North Pacific, Station 235, surface.

6. _Microcubus amphispyris_, n. sp. (Pl. 94, fig. 10).

Four gates (two mitral and two basal) simple; eight lateral gates (four superior and four inferior) fenestrated by arachnoidal irregular threads. Frontal ring convex, not constricted by the equatorial ring. Basal ring with six short conical divergent feet (two sagittal and four lateral). Sagittal ring with an apical horn. (Resembles some #Spyroidea#.)

_Dimensions._--Length of the shell 0.08, breadth 0.12.

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

Genus 429. _Octotympanum_,[48] n. gen.

_Definition._--#Tympanida# with two bisected horizontal rings converging laterally and connected in the lateral poles of the equatorial plane. Eight large gates.

The genus _Octotympanum_ differs from all other Tympanida in the peculiar connection of the two horizontal rings, which here reach the maximum of their growth, converge towards the equator, and come into direct contact on the poles of the equatorial transverse axis. Therefore the shell appears to be composed of three complete vertical meridian rings, the middle of which (the sagittal ring) is free and simple; the two other (subfrontal) rings are grown together at the lateral poles of the equatorial plane; commonly at these poles there arises (on the right and left) a strong lateral spine, and the base of this (or the crossing point of the two touching horizontal rings) corresponds to the remnants of the shortened frontal ring. The crown-shaped shell exhibits therefore eight large gates; two superior (mitral) and two inferior basal gates, two larger anterior (facial) and two larger posterior (occipital) gates. Seen from the lateral poles {1000}only two of the former are visible, and represent the figure of a vertical 8; seen from the sagittal poles, only two of the latter are visible, in the form of a larger horizontal 8. _Octotympanum_ may be derived from _Acrocubus_ by reduction of the lateral frontal ring to a knot (or cross-point), and by maximal extension of the two parallel horizontal rings, which become curved downwards laterally and touch at the lateral poles.

1. _Octotympanum octospinum_, n. sp.

Mitral gates smaller than the basal gates; upper halves of the two lateral meridional rings smaller than the lower halves. Rings with few scattered thorns; four larger simple spines, nearly horizontal in the equatorial plane; two on the poles of the sagittal axis, two on the poles of the lateral axis. Four other large spines, nearly vertical at the four lowermost basal points of the subfrontal rings.

_Dimensions._--Length of the shell 0.09, breadth 0.12.

_Habitat._--Central Pacific, Station 273, depth 2350 fathoms.

2. _Octotympanum octonarium_, n. sp. (Pl. 94, fig. 3).

Mitral gates of the same size as the basal gates; upper and lower halves of the meridional rings of equal size. Rings armed with numerous stout, thorny, simple and forked spines; four very large branched spines in the equatorial plane, nearly horizontal, two on the poles of the sagittal, two on the poles of the transverse axis, the latter curved downwards.

_Dimensions._--Length of the shell 0.1, breadth 0.2.

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

3. _Octotympanum arborescens_, n. sp.

Mitral gates smaller than the basal gates, the lower halves of the lateral rings being broader than the upper halves. Rings very spiny, armed with numerous branched spines; ten very large arborescent spines on the lateral rings; two on the poles of the transverse axis, eight on the prominent corners of the diagonal axes (four upper and four lower); sagittal ring thorny.

_Dimensions._--Length of the shell 0.8, breadth 0.16.

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

4. _Octotympanum cervicorne_, n. sp. (Pl. 94, fig. 2).

Mitral and basal gates of equal size, the lower and upper halves of the two crossed lateral rings being nearly equal. Rings armed with numerous branched spines; two very large spines, similar to the antlers of a deer, on the poles of the lateral axis; their branches about half as large as the whole shell. Sagittal ring nearly smooth, with few small thorns on the poles of the main axis.

_Dimensions._--Length of the shell 0.16, breadth 0.2.

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

{1001}Genus 430. _Tympaniscus_,[49] n. gen.

_Definition._--#Tympanida# with two bisected horizontal rings, which are connected by three vertical meridional rings (or six columellæ).

The genus _Tympaniscus_ and the following _Tympanidium_ differ from all preceding Tympanida in the multiplication of the vertical columellæ, which connect the two horizontal bisected rings. _Tympaniscus_ exhibits six columellæ, which may be regarded as halves of three meridional rings. Only one, however, of these is complete (the sagittal ring, which bisects the mitral and the basal ring); the two other meridional rings (placed in diagonal planes) may be regarded as produced by lateral bifurcation of an incomplete frontal ring; this appears to be already foreshadowed in the preceding _Octotympanum_.

1. _Tympaniscus corona_, n. sp.

Basal ring larger than the mitral ring, with numerous irregular short spines but without descending feet. Six columellæ strongly curved, in the lower half thicker and thorny; the four lateral much thinner than the two sagittal. Apex of the jointed sagittal ring with a conical horn.

_Dimensions._--Length of the shell 0.08, breadth 0.11.

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

2. _Tympaniscus bipes_, n. sp.

Basal ring of about the same size as the mitral ring, with two lateral descending feet, which are parallel and vertical, simple, without spur, half as long as the sagittal ring. The latter is ovate, twice as long and broad as the two circular frontal rings.

_Dimensions._--Breadth of the shell 0.14, length 0.09.

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

3. _Tympaniscus dipodiscus_, n. sp. (Pl. 94, figs. 5, 6).

Basal ring smaller than the mitral ring, with two lateral descending feet, which are nearly parallel and vertical, obtuse, about as long as the sagittal ring, and which bear on the outside a horizontal spur. Sagittal ring hexagonal, very stout, with a caudal appendix at the base, and with a small sagittal arch, bearing a stout spinulate double knob at the apex. The two subfrontal rings depressed, twice as broad as high, on the outside thorny, connected at each side in the lower half by a double transverse arch.

_Dimensions._--Breadth of the shell 0.18, length 0.08 (with spines 0.18).

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

{1002}4. _Tympaniscus tripodiscus_, n. sp. (Pl. 94, fig. 7).

Basal ring of about the same size as the mitral ring, with three large and stout divergent feet (two lateral and one caudal). Sagittal ring stout, broader than the two spiny subsagittal rings; the upper part of the latter bears two ascending arches, which are united with the large apical horn arising from the apex of the sagittal ring. Six columellæ nearly vertical, slightly curved on the outside; their lateral edges armed with numerous simple or forked spines.

_Dimensions._--Breadth of the shell 0.1, total length 0.12.

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

5. _Tympaniscus quadrupes_, n. sp. (Pl. 94, fig. 4).

Basal ring larger than the mitral ring, with four short vertical three-sided pyramidal feet of equal size (two lateral and two sagittal); the feet are three-sided pyramidal, with finely denticulate median edges, and bear at the base a horizontal thorny spur. Sagittal ring at the apex with a very large cross-shaped horn, bearing two horizontal thorny lateral branches; the horn is supported by an anterior and a posterior arch. Six columellæ strongly curved and dentate.

_Dimensions._--Breadth of the shell 0.15, length 0.12.

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

6. _Tympaniscus fibula_, Haeckel.

_Ceratospyris fibula_, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss.
Berlin, p. 66, Taf. xx. fig. 3.

Basal ring of about the same size as the mitral ring, with six large conical descending feet; these are prolongations of the six columellæ, nearly of the same length, slightly divergent, and bear at the base a strong horizontal spur. The six columellæ are strongly curved, spiny, and bear above small arches, which are united with the apical horn of the sagittal ring.

_Dimensions._--Breadth of the shell 0.09, height 0.05; total height 0.09.

_Habitat._--Fossil in Barbados.

7. _Tympaniscus dodecaster_, n. sp.

Basal ring of the same size as the mitral ring, with six very large and six alternating smaller, slightly divergent, curved feet, about as long an the shell; the larger are the basal prolongations of the six curved, thorny columellæ. Mitral ring spiny, not overgrown with arches, as in the similar preceding species.

_Dimensions._--Length of the shell 0.08, breadth 0.12.

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

{1003}Genus 431. _Tympanidium_,[50] Haeckel, 1881, Prodromus, p. 447.

_Definition._--#Tympanida# with two bisected horizontal rings, which are connected by four vertical meridional rings (or eight columellæ).

The genus _Tympanidium_ differs from all other Protympanida in the development of four vertical rings, placed in four different meridional planes. Only one of these is complete, the primary sagittal ring; the three others are incomplete, inserted on the two horizontal rings, which are formed by the paired mitral and basal branches of the sagittal ring; one of these three lies in the frontal plane, the two others in diagonal meridional planes (between the frontal and sagittal). The shell therefore exhibits twelve large gates, four of which are horizontal (the two superior mitral and the two inferior basal gates); the eight others are vertical, separated by the eight columellæ, or the halves of the four meridional rings. In the subgenus _Tympanomma_ the number of gates amounts to sixteen, the four lateral gates being bisected by an incomplete equatorial ring. The genus _Tympanidium_ may be derived from _Acrocubus_ by development of the two diagonal rings (between the frontal and sagittal).

Subgenus 1. _Tympanura_, Haeckel.

_Definition._--Shell with twelve gates; the four lateral gates simple.

1. _Tympanidium foliosum_, n. sp. (Pl. 94, fig. 1).

Shell with twelve gates; the four lateral gates simple. Basal gates of the shell larger than the mitral gates. Sagittal ring ovate, much smaller than the three other meridional rings, which are armed with numerous large elegant spines, bearing a lanceolate leaf on a thin pedicle. The sagittal ring bears a bunch of similar spines only at the apex, and on both poles of the sagittal axis (in the equator) a single forked spine, with two thorny branches.

_Dimensions._--Length of the shell 0.09 (with spines 0.25), breadth 0.15.

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

2. _Tympanidium spinosum_, n. sp.

Shell with twelve gates; the four lateral gates simple. Basal and mitral gates of about equal size. Shell similar to that of the preceding species, differing mainly in the different armature; the numerous spines, covering the convex outside of the four vertical rings, are not leaf-shaped, but conical, short, of very variable size, partly simple, partly irregularly branched.

_Dimensions._--Length of the shell 0.08 (with spines 0.2), breadth 0.12.

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

{1004}Subgenus 2. _Tympanomma_, Haeckel.

_Definition._--Shell with sixteen gates (eight median gates, paired on both sides of the sagittal ring, and eight lateral gates, paired on both sides of the frontal ring). The four lateral gates of the two preceding species (subgenus _Tympanura_) are bisected in _Tympanomma_ by the lateral parts of an incomplete equatorial ring.

3. _Tympanidium binoctonum_, n. sp. (Pl. 94, fig. 18).

Shell with sixteen gates; the four lateral gates double, bisected by the lateral parts of an incomplete equatorial ring. Basal gates nearly rectangular, of the same breadth as the pentagonal mitral gates, but twice as long. Equatorial outline of the shell (seen in fig. 18 from the apical pole) octagonal. All rods of the shell thin, smooth.

_Dimensions._--Transverse axis of the shell 0.1, sagittal axis 0.06.

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