Chapter XI: Literature and History (24)

Both arms equal, in the proximal half simple, in the distal half forked, with irregular septa; distal end of each arm blunt, without a terminal spine, smaller than the basal breadth of the arm. Axis of the branches straight. (Resembles _Dicranastrum furcatum_, Pl. 47, fig. 2, but without lateral arms.)

_Dimensions._--Radius of each arm 0.15, basal breadth 0.03; terminal breadth of each branch 0.02.

_Habitat._--Pacific, central area, Station 263, depth 2650 fathoms.

{522}Subgenus 2. _Amphirrhopella_, Haeckel.

_Definition._--Both opposite arms of similar size and form, with terminal spines.

3. _Amphirrhopalum bigeminum_, n. sp.

Both arms equal, in the proximal larger half simple, in the distal smaller half forked; each branch triangular, with a strong conical terminal spine. Axis of the branches straight. (Resembles _Dicranastrum cornutum_, Pl. 45, fig. 2, but without lateral arms.)

_Dimensions._--Radius of the arms (without spines) 0.15, basal breadth 0.03; breadth of the bifurcation 0.08.

_Habitat._--South Pacific, Station 284, surface.

4. _Amphirrhopalum echinatum_, n. sp. (Pl. 45, fig. 10).

Both arms equal, in the proximal smaller half simple, nearly square, in the distal larger half forked; the branches thorny, armed at the end with numerous spines, one larger on the terminal pole of the concavely curved arm-axis.

_Dimensions._--Radius of the arms (without spines) 0.15, basal breadth 0.05; breadth of branches 0.03.

_Habitat._--Pacific, central area, Station 266, depth 2750 fathoms.

Subgenus 3. _Amphirrhopoma_, Haeckel.

_Definition._--Both opposite arms of different size or form, without terminal spines.

5. _Amphirrhopalum ypsilon_, n. sp.

Both arms very different. Larger arm simple, egg-shaped, twice as long as broad; smaller arm in the basal half simple, nearly square, in the distal half forked; both branches egg-shaped, blunt. (Resembles _Amphicraspedum wyvilleanum_, Pl. 45, fig. 12, but wants the patagium.)

_Dimensions._--Radius of the larger simple arm 0.18, breadth 0.09; radius of the smaller forked arm 0.15, breadth of its branches 0.05.

_Habitat._--Indian Ocean, Madagascar, Rabbe, surface.

Genus 226. _Amphicraspedum_,[265] Haeckel, 1881, Prodromus, p. 460.

_Definition._--#Porodiscida# with two chambered arms, opposite in one axis, connected by a lateral patagium; one arm or both forked at the distal end.

The genus _Amphicraspedum_ exhibits the same bifurcation of the arms as does _Amphirrhopalum_, but differs from this ancestral form in the development of a patagium, an external connecticulum between the arms, which envelops the disk totally or partially.

{523}Subgenus 1. _Amphicraspedon_, Haeckel.

_Definition._--Both arms of equal size and form, without terminal spines of the branches.

1. _Amphicraspedum maclaganium_, n. sp. (Pl. 45, fig. 11).

Both arms equal, in the proximal half simple, in the distal half forked, with six to seven transverse septa; distal end of each branch rounded, blunt, somewhat broader than the base of the whole arm. Divergent axes of both branches concavely curved. Patagium incomplete, with elliptical perimeter, enveloping only the middle part of the shell. I call this interesting species in honour of Miss Nellie Maclagan, the learned translator of several zoological papers from German into English.

_Dimensions._--Radius of each arm 0.25, basal breadth 0.07; distal breadth of each branch 0.08; equatorial breadth of the patagium 0.25.

_Habitat._--North Atlantic, off Halifax, Station 50, surface.

Subgenus 2. _Amphicraspedina_, Haeckel.

_Definition._--Both arms of different size or form, without terminal spines on the branches.

2. _Amphicraspedum wyvilleanum_, n. sp. (Pl. 45, fig. 12).

Both arms different. Larger arm simple, egg-shaped, with eleven convex joints, one and a half times as long as broad; smaller arm in the basal half simple, triangular, with six cap-like joints, in the distal half forked; both branches egg-shaped, with five joints and blunt ends. Patagium nearly complete with four to five concave chamber-rows. Called in honour of Sir C. Wyville Thomson.

_Dimensions._--Radius of the larger simple arm 0.18, breadth 0.08; radius of the smaller forked arm 0.16; breadth of the branches 0.05; transverse breadth of the patagium 0.2.

_Habitat._--South Atlantic, Station 333, surface.

Subgenus 3. _Amphicraspedula_, Haeckel.

_Definition._--Both arms of different size or form, with terminal spines of the branches.

3. _Amphicraspedum murrayanum_, n. sp. (Pl. 44, fig. 10).

_Amphymenium murrayanum_, Haeckel, 1879, MS. et Atlas (pl. xliv. fig.
10).

Both arms different in size; the larger one and a half times as long and broad as the smaller. Both arms triangular, forked at the broader distal end, with two very strong, conical, divergent, {524}straight terminal spines. Patagium broad, incomplete, with circular perimeter. Called in honour of my friend Dr. John Murray.

_Dimensions._--Radius of the larger arm (including the spines) 0.24, of the smaller 0.16; distance of the terminal points of the former 0.18, of the latter 0.09; diameter of the patagium 0.2.

_Habitat._--North Atlantic, Færöe Channel, Gulf Stream, surface, John Murray.

Genus 227. _Dictyastrum_,[266] Ehrenberg, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 830.

_Definition._--#Porodiscida# with three simple, undivided, chambered arms, without a patagium; triangular shell regular, with three equal arms and three equal angles.

The genus _Dictyastrum_ is the simplest form of the Trigonastrida, or of the Porodiscida, in which the margin of the central disk is furnished with three chambered arms. In _Dictyastrum_ these are quite simple and regular, without a patagium, separated by equal angles, so that the whole shell represents a regular, equilateral triangle, if we connect the distal points of the arms by lines. The genus _Dictyastrum_, founded by Ehrenberg in 1860, differs from his _Rhopalodictyum_--after his own diagnosis--only by an insignificant difference in the form of the simple arms, which is scarcely a specific character. I therefore apply this name here in the above amended sense, seeing that the only figured species of Ehrenberg (_Dictyastrum angulatum_) occurs in two different, but externally very similar forms: one of these is a true Porodiscid (_Dictyastrum_) with two porous covering-plates and concentric rings; the other is a true Spongodiscid (_Rhopalodictyum_) with quite spongy, irregular network, and is probably identical with the _Rhopalodictyum truncatum_ of Ehrenberg.

Subgenus 1. _Dictyastrella_, Haeckel.

_Definition._--Arms with blunt ends, without terminal spines.

1. _Dictyastrum angulatum_, Ehrenberg.

_Dictyastrum angulatum_, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss.
Berlin, p. 289, Taf. viii. fig. 18.

Arms nearly square with straight edges, towards the truncated end a little broader, about the same diameter as the triangular central disk. The figure of Ehrenberg seems to represent a Spongodiscid (_Rhopalodictyum angulatum_), but in the same locality (Philippine Sea) occurs also a true _Dictyastrum_ of quite the same form, but with three to four concentric rings of the central disk, and with jointed arms.

{525}_Dimensions._--Radius of each arm (length from the centre to the distal end) 0.13; breadth of the truncated end 0.1.

_Habitat._--Tropical Pacific, Philippine Sea, Station 200, depth 250 fathoms.

2. _Dictyastrum bandaicum_, Haeckel.

_Rhopalastrum bandaicum_, Harting, 1863, Mikr. Fauna Banda-Zee, p. 16,
Taf. iii. fig. 45.

Arms nearly square, with convex edges, in the middle a little broader than at both ends, about half the diameter of the central disk. Differs from the nearly allied preceding species by the half size of the arms and the convex edges.

_Dimensions._--Radius of each arm 0.12, its greatest breadth 0.07.

_Habitat._--Tropical Pacific, Banda Sea, Harting.

3. _Dictyastrum hexagonum_, n. sp. (Pl. 43, fig. 7).

_Rhopalastrum hexagonum_, Haeckel, 1880, Atlas (pl. xliii. fig. 1).

Arms nearly triangular, one and a third times as broad at the distal end as long, and three times as broad as at the base. Central disk about the same diameter. In each arm six simple broad chambers. If we connect the six corners of the truncated distal ends by straight lines, we get a regular hexagon.

_Dimensions._--Radius of each arm 0.17, basal breadth 0.06, terminal breadth 0.17.

_Habitat._--Equatorial Atlantic, Station 347, surface.

4. _Dictyastrum trirrhopalum_, n. sp.

Arms club-shaped, five times as long as broad at the base, at the thickened end three times as broad as at the base. Diameter of the central disk equals half the length of the arms. (Similar to _Rhopalastrum malleus_, Pl. 43, fig. 1, but with three equal angles and much smaller disk.)

_Dimensions._--Radius of each arm 0.25, basal breadth 0.04, distal breadth 0.12.

_Habitat._--Pacific, central area, Station 273, surface.

Subgenus 2. _Dictyastromma_, Haeckel.

_Definition._--Arms on the distal end provided with terminal spines.

5. _Dictyastrum trispinosum_, n. sp. (Pl. 43, fig. 5).

_Rhopalastrum trispinosum_, Haeckel, 1881, Prodromus et Atlas (pl. xliii.
fig. 5).

Arms trapezoid, at the rounded distal end twice as broad as at the base, with a strong and short, conical, terminal spine. Diameter of the circular central disk about equal to the length and the greatest breadth of the arms.

_Dimensions._--Radius of each arm 0.15, basal breadth 0.06, distal breadth 0.11.

_Habitat._--Equatorial Atlantic, Station 347, surface.

{526}6. _Dictyastrum triactis_, Ehrenberg.

_Dictyastrum triactis_, Ehrenberg, 1872, Monatsber. d. k. preuss. Akad.
d. Wiss. Berlin, p. 306.

Arms rectilinear, four times as long as broad, with parallel edges, pointed at the distal end, with a short terminal spine. Diameter of the circular central disk equal to double the breadth of the arms.

_Dimensions._--Radius of each arm 0.2, breadth 0.04.

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

7. _Dictyastrum aculeatum_, n. sp.

Arms lanceolate, three times as long as broad, twice as broad in the middle as at either end, with thorny surface and numerous conical terminal spines, one very large in the radius. Central disk triangular, about as broad as the arms. (Resembles _Rhopalastrum arcticum_, Pl. 43, fig. 6, but differs by the equal angles and the triangular disk.)

_Dimensions._--Radius of each arm 0.2, breadth 0.06.

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

Genus 228. _Rhopalastrum_,[267] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.

_Definition._--#Porodiscida# with three simple undivided, chambered arms, without a patagium; triangular shell bilateral, one odd arm opposite to the odd angle between two paired arms.

The genus _Rhopalastrum_, founded by Ehrenberg (1847) with a very insufficient diagnosis, is here retained for those Trigonastrida that agree in the generic characters with the only species figured by him, viz., _Rhopalastrum lagenosum_ (compare my Monograph, 1862, p. 500). It comprises, therefore, such Euchitonida as agree with the preceding _Dictyastrum_ in the simple form of the three arms and the absence of a patagium, but differ from it in the different size of the three angles, and often also in the divergent form and size of the three arms; one odd arm is opposite to the odd angle between the two paired arms.

Subgenus 1. _Rhopalastrella_, Haeckel.

_Definition._--Arms with blunt ends, without terminal spines.

1. _Rhopalastrum truncatum_, Haeckel.

_Rhopalastrum truncatum_, Haeckel, 1862, Monogr. d. Radiol., p. 500, Taf.
xxix. fig. 6.

Distance of both paired arms about half as large as their distance from the odd arm. All three arms nearly of the same form and size, very short and broad; their breadth nearly equals that of {527}the roundish central disk, whilst their length reaches only one-fourth of it. End of the arms convex rounded, without spines.

_Dimensions._--Radius of each arm 0.1, breadth 0.15.

_Habitat._--Mediterranean (Messina), Atlantic (Canary Islands), surface, Haeckel.

2. _Rhopalastrum pistillum_, Stöhr.

_Rhopalastrum pistillum_, Stöhr, 1880, Palæontogr., vol. xxvi. p. 110,
Taf. v. fig. 4.

Distance between the paired arms about two-thirds as large as their distance from the odd arm. All three arms nearly of the same form and size, about three times as long as the diameter of the central disk, at the base one-third as broad as at the convex rounded end, without spines. Stöhr has only observed a fragment with one arm; some perfect specimens, which I found in the Caltanisetta-rock, exhibited nearly the same form as _Rhopalastrum malleus_ (Pl. 43, fig. 1), but differ from this by the smaller disk, the broader arms, and the smaller angle between the paired arms.

_Dimensions._--Radius of all three arms 0.2; basal breadth of each arm 0.045, terminal breadth 0.13.

_Habitat._--Fossil in Tertiary rocks of Sicily, Grotte (Stöhr), Caltanisetta (Haeckel).

3. _Rhopalastrum malleus_, n. sp. (Pl. 43, fig. 1).

Distance between the paired arms one and a third times as large as their distance from the odd arm. All three arms nearly of the same form and size, hammer-shaped, three times as broad at the truncated distal end as at the base. Central disk broader than the arms.

_Dimensions._--Radius of each arm 0.25, basal breadth 0.05, distal breadth 0.15.

_Habitat._--South Atlantic, Station 325, surface.

4. _Rhopalastrum lagenosum_, Ehrenberg.

_Rhopalastrum lagenosum_, Ehrenberg, 1847, Monatsber. d. k. preuss. Akad.
d. Wiss. Berlin, p. 43; Mikrogeol., 1854, Taf. xxii. fig. 22.

_Rhopalastrum lagenosum_, Haeckel, 1862, Monogr. d. Radiol., p. 501.

_Flustrella bilobata_, Ehrenberg, 1844, Monatsber. d. k. preuss. Akad. d.
Wiss. Berlin, p. 81.

_Haliomma lagena_, Ehrenberg, 1840, Monatsber. d. k. preuss. Akad. d.
Wiss. Berlin, p. 200.

Distance between the paired arms one and a half times as great as their distance from the odd arm. All three arms nearly of the same form and size, about one and a half times as long as the diameter of the central disk, at the base half as broad as at the convex rounded end, without spines. Ehrenberg has only observed a fragment with two paired arms; some perfect specimens with three arms, observed by me, differed from the nearly allied _Rhopalastrum pistillum_ (from the same locality) by the larger disk, the broader arms, and the larger unpaired angle.

_Dimensions._--Radius of all three arms 0.2, basal breadth 0.05, terminal breadth 0.09.

_Habitat._--Fossil in Tertiary rocks of Sicily, Caltanisetta.

{528}5. _Rhopalastrum ypsilinum_, n. sp. (Pl. 43, fig. 2).

Distance between the paired arms half as large as their distance from the odd arm. All three arms at the concavely curved distal end three times as broad as at the narrow base. Odd arm twice as long and broad as the paired arms. Central disk smaller than the latter. No terminal spines.

_Dimensions._--Radius of the odd arm 0.32, of the paired arms 0.2; terminal breadth of the former 0.2, of the latter 0.12.

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

6. _Rhopalastrum clavatum_, n. sp.

Distance between the paired arms half as large as their distance from the odd arm. All three arms club-shaped, at the thickened, nearly spherical, distal end three times as broad as at the narrow base. Odd arm nearly twice as long and broad as the paired arms. Central disk equal to the distal knob of the latter. No terminal spines.

_Dimensions._--Radius of the odd arm 0.3, of the paired arms 0.18; distal breadth of the former 0.16, of the latter 0.08.

_Habitat._--South Atlantic, Station 332, surface.

7. _Rhopalastrum irregulare_, n. sp. (Pl. 43, fig. 8).

Distance between all three arm-points different. All three arms cylindrical, nearly of the same length, but of different form, irregularly curved, about four times as long as broad, with blunt ends.

_Dimensions._--Length of each arm about 0.2, breadth 0.05.

_Habitat._--Antarctic Sea, Station 157, depth 1950 fathoms.

Subgenus 2. _Rhopalastromma_, Haeckel.

_Definition._--Arms on the distal end provided with one or more terminal spines.

8. _Rhopalastrum martellum_, n. sp.

Distance between all three arm-points nearly the same; but the odd arm is one and a half times as large as both paired arms, and is perpendicular to the common axis of the latter, therefore the shell has the form of a hammer. Each arm is twice as broad at the distal end as at the base, and armed with a conical terminal spine; the latter is vertical in the odd arm, horizontal in the paired arms.

_Dimensions._--Radius of the odd arm 0.3, of the paired arms 0.15; distal breadth of the former 0.12, of the latter 0.06.

_Habitat._--North Atlantic, Station 354, surface.

{529}9. _Rhopalastrum triceros_, n. sp. (Pl. 43, fig. 4).

Distance between the paired arms about two-thirds as large as their distance from the odd arm. All three arms club-shaped, three times as broad at the thickened distal part as at the base, and armed with one single, conical, terminal spine. Odd arm of the same breadth, but twice as long as the paired arms.

_Dimensions._--Radius of the odd arm 0.35, of the paired arms 0.2; basal breadth 0.04, distal breadth 0.12.

_Habitat._--Pacific, central area, Station 274, surface.

10. _Rhopalastrum hexaceros_, n. sp. (Pl. 43, fig. 3).

Distance between the paired arms equals four-fifths of their distance from the odd arm. All three arms nearly of the same size, about square, a little broader at the truncated distal end, which is armed at both corners with a strong, conical, radial spine.

_Dimensions._--Radius of each arm 0.2, basal breadth 0.1, distal breadth 0.12.

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

11. _Rhopalastrum arcticum_, n. sp. (Pl. 43, fig. 6).

Distance between the paired arms half as large as their distance from the odd arm, which is a little larger. All three arms of the same form, lanceolate, twice to three times as long as broad, twice as broad in the middle as at either end. Each arm with twelve to fourteen transverse septa, at the distal end with a bunch of conical spines, and one single, very large, pyramidal, terminal spine.

_Dimensions._--Radius of each arm (without spine) 0.17, greatest breadth of it 0.05 to 0.06.

_Habitat._--Arctic Ocean, lat. 83° 19' N., North Polar expedition of the "Alert."

Genus 229. _Hymeniastrum_,[268] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.

_Definition._--#Porodiscida# with three simple, undivided, chambered arms, connected by a patagium; triangular shell regular, with three equal arms and three equal angles.

The genus _Hymeniastrum_ was founded by Ehrenberg (1847) with a very incomplete diagnosis, and hitherto known only by one single species, figured by him as _Hymeniastrum pythagoræ_ (Mikrogeol., 1854, Taf. xxxvi. fig. 31). This form occurs in two different states, externally quite identical; in one state the central disk (as figured, _loc. cit._), is a simple lens or hollow disk, containing a medullary shell or "central chamber"; in the other state the central disk is composed of two concentric rings surrounding the "central chamber." We retain here the name _Hymeniastrum_ for this latter state, {530}expressed in the diagnosis given above, and call the former state (the Coccodiscid) _Hymenactura_ (compare above, p. 473). One practical advantage, obtained in this way, is that all genera of #Discoidea# ending with "_-astrum_" belong to the Porodiscida. _Hymeniastrum_ differs from _Dictyastrum_ by the possession of a patagium, and from _Euchitonia_ by the equal size of the angles and the arms.

Subgenus 1. _Hymenastrella_, Haeckel.

_Definition._--Arms with blunt ends, without terminal spines.

1. _Hymeniastrum leydigii_, Haeckel.

_Euchitonia leydigii_, Haeckel, 1862, Monogr. d. Radiol., p. 510, Taf.
xxxi. figs. 4, 5.

_Hymeniastrum leydigii_, Haeckel, 1881, Prodromus, p. 460.

_Histiastrum trinacrium_, Haeckel, 1860, Monatsber. d. k. preuss. Akad.
d. Wiss. Berlin, p. 843.

Arms twice as long as broad, two-thirds as broad at the base as at the blunt, nearly truncated distal end. Patagium nearly complete, perfectly filling out the interbrachial spaces, with six to seven regular, concave chamber-rows; only the truncate terminal faces of the arms free.

_Dimensions._--Radius of the arms 0.15 to 0.18, breadth on their base 0.04 to 0.05, on their broadest distal end 0.06 to 0.07.

_Habitat._--Cosmopolitan; Mediterranean, Atlantic, Pacific, surface. Very common and variable.

2. _Hymeniastrum köllikeri_, Haeckel.

_Euchitonia köllikeri_, Haeckel, 1862, Monogr. d. Radiol., p. 511, Taf.
xxxi. figs. 6, 7.

_Hymeniastrum köllikeri_, Haeckel, 1881, Prodromus, p. 460.

Arms nearly lanceolate, twice as long as broad, half as broad at the base as at the broadest middle part, pointed at the distal end. Patagium complete, enveloping the whole triangular disk, with irregular chamber-rows.

_Dimensions._--Radius of the arms 0.2, breadth on their base 0.04, on the broadest part 0.08.

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

3. _Hymeniastrum gümbelii_, Haeckel.

_Stylactis gümbelii_, Stöhr, 1880, Palæontogr., vol. xxvi. p. 112, Taf.
vi. fig. 1.

Arms twice as long as broad, nearly twice as broad in the circular distal half as in the square proximal half. Patagium incomplete, circular, with eight to nine convex chamber-rows, enveloping only the proximal square halves of the arms; the lenticular distal halves remain free.

_Dimensions._--Radius of the arms 0.14; breadth at the base 0.04, at the distal lenticular part 0.07.

_Habitat._--Fossil in the Tertiary rocks of Sicily, Grotte, Stöhr.

{531}4. _Hymeniastrum euclidis_, n. sp. (Pl. 43, fig. 13).

_Euchitonia euclidis_, Haeckel, 1881, Prodromus, p. 460 et Atlas (pl.
xliii. fig. 13).

Arms one and a half times as long as broad, club-shaped, three times as broad in the oval distal part as in the narrow square proximal part. Patagium complete, enveloping the whole triangular disk, and also the convex ends of the arms, with eight to nine convex chamber-rows.

_Dimensions._--Radius of the arms 0.18; breadth at the base 0.025, at the broadest distal part 0.07.

_Habitat._--Pacific, central area, Stations 270 to 274, surface.

5. _Hymeniastrum pythagoræ_, Ehrenberg.

_Hymeniastrum pythagoræ_, Ehrenberg, 1854 (_partim_), Mikrogeol., Taf.
xxxvi. fig. 31.

Arms nearly as broad as long, two-thirds as broad at the base as at the truncated distal end. Patagium incomplete, enveloping only the basal half of the arms, with four to five rectilinear parallel chamber-rows. (This form has the greatest resemblance to the figure given by Ehrenberg, _loc. cit._, but differs by the central disk, which is composed of two concentric rings surrounding the small central chamber; compare above _Hymenactura pythagoræ_, p. 474.)

_Dimensions._--Radius of the arms 0.18; breadth at the base 0.08, at the truncated end 0.12.

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

6. _Hymeniastrum archimedis_, n. sp.

Arms nearly triangular, at the truncated, slightly convex end three times as broad as at the narrow base and one and a third times as broad as long; each arm with six simple chambers. Patagium incomplete, enveloping only the basal half of the arms with three to four convex chamber-rows. (Differs from _Rhopalastrum hexagonum_, Pl. 43, fig. 7, only by the patagium.)

_Dimensions._--Radius of the arms 0.2; breadth at the base 0.07, at the truncated end 0.2.

_Habitat._--Pacific, central area, Station 274, surface.

Subgenus 2. _Hymenastromma_, Haeckel.

_Definition._--Arms at the distal end provided with radial spines.

7. _Hymeniastrum ternarium_, Haeckel.

_Histiastrum ternarium_, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss.
Berlin, p. 76, Taf. xxiv. fig. 2.

Arms two and a half times as long as broad, three times as broad at the truncated end as at the narrow base, with a strong, conical, radial spine at the end. Patagium incomplete, enveloping only the basal half of the arms, with three to four convex chamber-rows.

_Dimensions._--Radius of the arms (without terminal spine) 0.02; breadth at the base 0.025, at the broadest terminal part 0.08.

_Habitat._--Fossil in the rocks of Barbados.

{532}8. _Hymeniastrum trigonarium_, n. sp.

Arms one and a half times as long as broad, a little broader at the rounded end than at the base, with three strong conical radial spines, one larger (in the radius of each arm) and two smaller on both sides of this. Patagium complete, with four to five rectilinear parallel chamber-rows, enveloping the whole arms (with exception of the terminal spines) and forming a perfect equilateral triangle.

_Dimensions._--Radius of the arms (without terminal spines) 0.18; breadth at the base 0.1, at the distal part 0.12.

_Habitat._--Pacific, central area, Station 268, depth 2900 fathoms.

Genus 230. _Euchitonia_,[269] Ehrenberg, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 831 (_sensu emendato_).

_Definition._--#Porodiscida# with three simple, undivided, chambered arms, connected by a patagium; triangular shell bilateral, one odd arm opposite to the odd angle between the two paired arms.

The genus _Euchitonia_, quite insufficiently characterised by Ehrenberg, was founded by him (1860) for one single species, afterwards (1872) described and figured as _Euchitonia furcata_. Retaining this species correctly as the type of this genus, I give to it here the above diagnosis. In my Monograph (1862, p. 503) I described seven Mediterranean species of _Euchitonia_. Three of these have in common the characters according to the present diagnosis: _Euchitonia mülleri_, _Euchitonia virchowii_, _Euchitonia beckmanni_; two others appertain (on account of the regular, not bilateral form) to _Hymeniastrum_, and two others (on account of the forked, not simple arms) to _Trigonastrum_. Afterwards (1880) three true fossil species of _Euchitonia_ were described by Stöhr (_Euchitonia cruciata_, _Euchitonia zittelii_, and _Euchitonia acuta_). Some species of this genus are cosmopolitan, and appertain to the most common and everywhere represented #Discoidea#.

Subgenus 1. _Stylactis_, Ehrenberg, 1872 (_loc. cit._).

_Definition._--Arms with blunt ends, without terminal spines.

1. _Euchitonia furcata_, Ehrenberg.

_Euchitonia furcata_, Ehrenberg, 1872, Monatsber. d. k. preuss. Akad. d.
Wiss. Berlin, p. 308; Abhandl. d. k. Akad. d. Wiss. Berlin, Taf. vi. iii.
fig. 6.

Distance between the paired arms about half as large as their distance from the odd arm. All three arms nearly of the same size and form, about twice as long as broad, one and a half times at the {533}blunt convex end as broad as at the base. Patagium incomplete, with concave chamber-rows and irregular network, only enveloping the basal half of the arms.

_Dimensions._--Radius of all three arms 0.15, basal breadth of each arm 0.04, terminal breadth 0.06.

_Habitat._--North Pacific, Californian Sea, depth 2600 fathoms, Ehrenberg.

2. _Euchitonia mülleri_, Haeckel.

_Euchitonia mülleri_, Haeckel, 1862, Monogr. d. Radiol., p. 508, Taf.
xxx. figs. 5-10.

_Euchitonia mülleri_, Stöhr, 1880, Palæontogr., vol. xxvi. p. 110, Taf.
v. fig. 5.

Distance between the paired arms about two-thirds as large as their distance from the odd arm, which is somewhat larger. Length of the arms equals two and a half times the breadth of the blunt convex end or five times the breadth of the base. Patagium with concave chamber-rows, nearly complete, enveloping the arms with exception of the terminal face. (This common species is very variable; compare my Monograph.)

_Dimensions._--Radius of all three arms 0.16 to 0.22, basal breadth 0.03 to 0.05, terminal breadth 0.06 to 0.08.

_Habitat._--Cosmopolitan; one of the most common #Discoidea# in all seas, on the surface as well as at different depths; also fossil in the Tertiary rocks of Barbados and Sicily.

3. _Euchitonia triangulum_, Haeckel.

_Stylactis triangulum_, Ehrenberg, 1872, Monatsber. d. k. preuss. Akad.
d. Wiss. Berlin, p. 320; Abhandl. d. k. Akad. d. Wiss. Berlin, Taf. viii.
fig. 9.

_Stylactis triangulum_, Stöhr, 1880, Palæontogr., vol. xxvi. p. 113, Taf.
vi. fig. 2.

Distance between the paired arms about two-thirds as large as their distance from the odd arm. All three arms nearly of the same size and form, about one and a half times as long as broad, nearly as broad at the base as at the blunt rounded end. Patagium incomplete, with convex chamber-rows, enveloping about two-thirds of the arms.

_Dimensions._--Radius of all three arms 0.15, basal breadth 0.05, terminal breadth 0.06.

_Habitat._--Pacific and Atlantic, Stations 253, 272, 354, surface; also fossil in the Tertiary rocks of Barbados and Sicily.

4. _Euchitonia cruciata_, Stöhr.

_Euchitonia cruciata_, Stöhr, 1880, Palæontogr., vol. xxvi. p. 111, Taf.
v. fig. 7.

Distance between the paired arms about one and a half times as large as their distance from the odd arm, which is a little larger. Length of the arm nearly equals twice the breadth, which is the same at the base and at the rounded blunt ends. Patagium incomplete, with concave chamber-rows, enveloping only the base of the arms, and forms between them three other smaller arms; therefore the whole shell forms six angles with six alternating arms.

_Dimensions._--Radius of all three arms 0.15, basal breadth 0.05.

_Habitat._--Fossil in Tertiary rocks of Sicily, Grotte, Stöhr.

{534}5. _Euchitonia lanceolata_, n. sp. (Pl. 43, fig. 9).

Distance between the paired arms about one and a third times as large as their distance from the odd arm. All three arms nearly of the same size and form, three times as long as broad, lanceolate, much broader in the middle part than at both ends; distal end cuspidate, but not spiny. Patagium incomplete, with convex chamber-rows, envelops about two-thirds of the arms.

_Dimensions._--Radius of all three arms 0.18, greatest breadth (in the width) 0.06.

_Habitat._--North Atlantic, Station 353, depth 2965 fathoms.

6. _Euchitonia zittelii_, Haeckel.

_Stylactis zittelii_, Stöhr, 1880, Palæontogr., vol. xxvi. p. 112, Taf.
v. fig. 8.

Distance between the paired arms about one-fifth as large as their distance from the odd arm. All three arms nearly of the same size and form, in the proximal half thinner and nearly square, in the distal half thicker and circular; the latter half twice to three times as broad as the former. Patagium incomplete, with convex chamber-rows, envelops only the proximal square half of the arms.

_Dimensions._--Radius of all three arms 0.15, basal breadth 0.03, terminal breadth 0.08.

_Habitat._--Fossil in Tertiary rocks of Sicily, Grotte, Stöhr.

7. _Euchitonia stöhrii_, n. sp. (Pl. 43, fig. 12).

Distance between the paired arms about four-fifths as large as their distance from the odd arm, which is one and a half times as long as the former. Arms about three times as long as broad, twice as broad in the distal half as in the proximal half, with rounded blunt ends. Patagium complete, with concave chamber-rows, enveloping the whole shell, also the ends of the arms.

_Dimensions._--Radius of the paired arms 0.2, of the odd arm 0.25; basal breadth 0.03, distal breadth 0.06.

_Habitat._--Fossil in the rocks of Barbados and of Nicobar Islands, Haeckel. Indian Ocean, Cocos Islands (Rabbe). Also in the Central Pacific, Station 267.

8. _Euchitonia beckmannii_, Haeckel.

_Euchitonia beckmannii_, Haeckel, 1862, Monogr. d. Radiol., p. 505, Taf.
xxxi. fig. 1.

Distance between the paired arms scarcely half as large as their distance from the odd arm, which is somewhat larger. Each arm with six simple broad chambers (without radial septa), the terminal chambers semilunar, convex, blunt, four times as broad as the first (basal) chamber. Patagium incomplete, with convex chamber-rows, enveloping the arms with exception of the broad blunt terminal face.

_Dimensions._--Radius of the paired arms 0.15, of the odd arm 0.18; basal breadth 0.02 to 0.03, terminal breadth 0.1 to 0.15.

_Habitat._--Mediterranean (Messina), Haeckel, surface.

{535}9. _Euchitonia virchowii_, Haeckel.

_Euchitonia virchowii_, Haeckel, 1862, Monogr. d. Radiol., p. 503, Taf.
xxx. figs. 1-4.

_Histiastrum fasciatum_, Haeckel, 1860, Monatsber. d. k. preuss. Akad. d.
Wiss. Berlin, p. 842.

Distance between the paired arms about half as large as their distance from the odd arm, which is somewhat larger. Each arm with six broad chambers, bisected by a radial septum; the terminal chamber convex, blunt, twice as broad as the basal chamber. Patagium incomplete, with concave chamber-rows, enveloping the arms with exception of the broad blunt terminal face.

_Dimensions._--Radius of the paired arms 0.15, of the odd arm 0.16; basal breadth 0.05, terminal breadth 0.1 to 0.12.

_Habitat._--Mediterranean (Messina), Atlantic (Canary Islands).

Subgenus 2. _Pteractis_, Ehrenberg, 1872 (_loc. cit._).

_Definition._--Arms provided with radial spines at the distal end.

10. _Euchitonia elegans_, Haeckel.

_Pteractis elegans_, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss.
Berlin, p. 299, Taf. viii. fig. 3.

Distance between the paired arms half as large as their distance from the odd arm. This latter is straight, while both the former are concavely curved towards the middle line. Arms five times as long as broad, at the distal end pointed and armed with a short conical terminal spine. Patagium nearly complete, enveloping four-fifths of the arms, with four to five concave chamber-rows.

_Dimensions._--Radius of the arms 0.2, breadth of them 0.03.

_Habitat._--Tropical Pacific, Philippine Sea, depth 3300 fathoms (Ehrenberg).

11. _Euchitonia carcinus_, n. sp. (Pl. 43, fig. 10).

Distance between the paired arms scarcely one-fourth as great as their distance from the odd arm. This latter is straight, twice as long as the former, which are concavely curved towards the middle line. The odd arm is three times as broad at the distal end as at the narrow base. The end of each arm is furnished with a strong triangular radial spine and a group of smaller spines. Patagium incomplete, with two to three concave chamber-rows, enveloping only the basal half of the arms.

_Dimensions._--Radius of the paired arms 0.15, breadth 0.03; radius of the odd arm 0.3, breadth on its base 0.02, on its distal end 0.07.

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

{536}12. _Euchitonia acuta_, Stöhr.

_Euchitonia acuta_, Stöhr, 1880, Palæontogr., vol. xxvi. p. 111, Taf. v.
fig. 6.

Distance between the paired arms two-thirds as great as their distance from the odd arm. All three arms nearly of the same size and form, two and a half times as long as broad, nearly lanceolate, at their broadest part one and a half times as broad as at their base. Patagium complete, enveloping the whole triangular disk, with five to six concave chamber-rows. In the figure of Stöhr the ends of the arms are simply pointed, while I find in the same fossil form a short terminal conical spine.

_Dimensions._--Radius of all three arms 0.14; breadth at the base 0.04, at the broadest part 0.06.

_Habitat._--Fossil in Tertiary rocks of Sicily, Grotte (Stöhr), Caltanisetta (Haeckel).

13. _Euchitonia ypsiloides_, Haeckel.

_Histiastrum ypsiloides_, Haeckel, 1860, Monatsber. d. k. preuss. Akad.
d. Wiss. Berlin, p. 843.

Distance between the paired arms two-thirds as great as their distance from the odd arm, which is somewhat larger. Length of the arms equals five times the breadth of the narrow base, which is half that of the distal end; this latter is armed with three to five short conical spines. Patagium complete, with six to seven concave chamber-rows, enveloping the whole arms with the exception of the terminal spines. (Differs from _Euchitonia mülleri_ almost solely by the possession of terminal spines.)

_Dimensions._--Radius of the arms 0.18 to 0.2; breadth at the base 0.04, at the distal end 0.08.

_Habitat._--Mediterranean (Messina), Atlantic (Canary Islands), surface.

14. _Euchitonia echinata_, n. sp. (Pl. 43, fig. 11).

Distance between paired arms three-fourths as great as their distance from the odd arm, which is somewhat larger. Arms one and a half times as long as broad, somewhat constricted in the middle part, armed at the rounded ends with numerous (thirty to forty) strong, conical spines. Patagium complete, with four or five rectilinear parallel chamber-rows, enveloping the whole of the arms with the exception of the spiny ends.

_Dimensions._--Radius of the arms (without spines) 0.2, breadth 0.06 to 0.09.

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

Genus 231. _Chitonastrum_,[270] Haeckel, 1881, Prodromus, p. 460.

_Definition._--#Porodiscida# with three forked, chambered arms, without a patagium. (Arms and angles between them either equal or unequal.)

The genus _Chitonastrum_ differs from its ancestral form, _Dictyastrum_, by the bifurcation of the distal ends of the arms. The few species of this genus are partly {537}regular (like _Dictyastrum_), partly bilateral (like _Rhopalastrum_). If the number of species increases much, these two subgenera may be separated into two genera: _Chitonastrella_ corresponding to the former, _Chitonastromma_ to the latter.

Subgenus 1. _Chitonastrella_, Haeckel.

_Definition._--All three arms of the same size and form, equidistant; fundamental form of the shell therefore an equilateral triangle.

1. _Chitonastrum triglochin_, n. sp.

All three arms equal and equidistant. Each arm has the form of an isosceles triangle, twice as high as broad; the truncated apex of the triangle is inserted into the large central disk, whilst its distal base (four times as broad) is divided by a deep incision (half as long as the arm). Each arm with ten to twelve joints, simple in its basal half, double in its distal half. Axes of the six branches straight. (Resembles _Trigonastrum regulare_, Pl. 43, fig. 16, but differs in the absence of a patagium.)

_Dimensions._--Radius of each arm 0.24, greatest breadth 0.11, basal breadth 0.03.

_Habitat._--Pacific, central area, Station 271, depth 2425 fathoms.

2. _Chitonastrum bathybium_, n. sp.

All three arms equal and equidistant, in the basal two-thirds simple, rectilinear, three times as long as broad, in the distal third forked, both branches equal, straight, blunt, half as broad as the basal part.

_Dimensions._--Radius of each arm 0.18, basal breadth 0.04, breadth of the branches 0.02.

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

Subgenus 2. _Chitonastromma_, Haeckel.

_Definition._--One odd arm different in size or form from the two other arms, which are paired; distance between them different; fundamental form of the shell therefore a bilateral, isosceles triangle.

3. _Chitonastrum jugatum_, n. sp. (Pl. 43, fig. 14).

_Dictyastrum jugatum_, Haeckel, 1881, Prodromus et Atlas (pl. xliii. fig.
14).

Arms very different; odd arm club-shaped, twice as long as broad, at the blunt distal end twice as broad as at the base; its axis is perpendicular to the common axis of both paired arms, which are only two-thirds as long, not so broad, and in the distal half divided into two branches; the anterior branch is straight, nearly horizontal, the posterior shorter and curved backwards.

{538}_Dimensions._--Radius of the odd arm 0.2, of the paired arms 0.15; distal breadth of the former 0.08, basal breadth 0.04; breadth of the paired arms 0.04.

_Habitat._--South Atlantic, Station 325, surface.

4. _Chitonastrum dicranoides_, n. sp.

All three arms in the basal half simple, nearly square, in the distal half forked; branches straight, blunt. Odd arm twice as large as the paired arms; angle between the latter larger than the angles between them and the odd arm. (The form of the arms resembles _Dicranastrum furcatum_, Pl. 47, fig. 2.)

_Dimensions._--Radius of the odd arm 0.24, of the paired arms 0.12; basal breadth 0.06.

_Habitat._--North Atlantic, Station 353, surface.

5. _Chitonastrum lyra_, n. sp. (Pl. 43, fig. 15).

_Dictyastrum lyra_, Haeckel, 1881, Prodromus et Atlas (pl. xliii. fig.
15).

All three arms forked and nearly of the same size, but different in form and position. The distance between the branches of the two paired arms is only one-fourth of the distance between them and the odd arm. Each arm in the basal two-thirds is simple, with eleven to twelve transverse septa, in the distal third forked, each branch with four to five transverse septa. The branches of each arm are curved convexly one to another, ending obtusely. The axis of the simple proximal part is straight in the odd arm, in the paired arms curved concavely towards the middle line. In the figured specimen, which I observed living in Portofino (in September 1880), the central chamber of the central disk and the first surrounding ring were filled with the nucleus of the cell; both external rings were filled (like all chambers of the arms) with pink oil-globules of the red central capsule. From the mantle, enveloping the shell, radiated innumerable fine pseudopodia (much too short in the figure), and between the two paired arms a long "sarcode-flagellum."

_Dimensions._--Radius of each arm 0.16; greatest breadth of the odd arm 0.04; basal breadth of the paired arms 0.02; distance of both branches of each arm 0.08.

_Habitat._--Mediterranean, Portofino, near Genoa, Haeckel.

Genus 232. _Trigonastrum_,[271] n. gen.

_Definition._--#Porodiscida# with three forked, chambered arms, connected by a patagium. (Arms and angles between them either equal or unequal.)

The genus _Trigonastrum_ differs from the preceding _Chitonastrum_, its ancestral form, in the development of a patagium between the arms. It bears therefore to the latter the same relation that _Euchitonia_ does to _Rhopalastrum_.

{539}Subgenus 1. _Trigonastrella_, Haeckel.

_Definition._--All three arms of the same size and form, equidistant; fundamental form of the shell therefore a regular, equilateral triangle.

1. _Trigonastrum regulare_, n. sp. (Pl. 43, fig. 16).

_Chitonastrum regulare_, Haeckel, 1881, Prodromus et Atlas (pl. xliii.
fig. 16).

All three arms equal and equidistant. Each arm has the form of an isosceles triangle, twice as high as broad, the truncated apex of which is inserted into the large circular central disk, whilst its distal base (four times as broad) is divided by a deep incision (half as long as the arm). Each arm with ten to twelve joints. Patagium between the arms nearly complete, spongy. (Differs from _Chitonastrum triglochin_ mainly in the possession of a patagium.)

_Dimensions._--Radius of each arm 0.24, greatest breadth 0.11, basal breadth 0.03; length of the sides of the regular triangle 0.45.

_Habitat._--Pacific, central area, Station 274, surface.

Subgenus 2. _Trigonastromma_, Haeckel.

_Definition._--One odd arm different in form or size from the other two arms, which are paired; distance between them different; fundamental form of the shell therefore an isosceles triangle.

2. _Trigonastrum krohnii_, Haeckel.

_Euchitonia krohnii_, Haeckel, 1862, Monogr. d. Radiol., p. 507.

All three arms different; distance between the two paired arms smaller than their distance from the odd arm. Odd arm with six joints. Each paired arm with seven joints, increasing in breadth towards the distal end. Odd arm and one paired arm forked at the end, the other paired arm simple. Patagium nearly complete. (The asymmetry in this form may perhaps be an individual anomaly, as also in _Myelastrum anomalum_, Pl. 47, fig. 9.)

_Dimensions._--Radius of the arms about 0.13 to 0.16, breadth 0.05 to 0.08.

_Habitat._--North Atlantic, Funchal, Madeira, Krohn, surface.

3. _Trigonastrum gegenbauri_, Haeckel.

_Euchitonia gegenbauri_, Haeckel, 1862, Monogr. d. Radiol., p. 506, Taf.
xxxi. figs. 2, 3.

Arms different; distance between the paired arms larger than their distance from the odd arm, which is one-third shorter. Odd arm egg-shaped, simple, with seven joints, undivided. Both paired arms equal, with ten joints, in the distal third forked. Patagium nearly complete.

_Dimensions._--Radius of the odd arm 0.14, greatest breadth 0.08; radius of the paired arms 0.2.

_Habitat._--Mediterranean (Messina), Haeckel, surface.

{540}Genus 233. _Stauralastrum_,[272] n. gen.

_Definition._--#Porodiscida# with four simple, undivided, chambered arms, without a patagium; quadrangular shell a regular cross, with four equal arms placed at right angles.

The genus _Stauralastrum_ is the most simple form of the Tessarastrida, or of those Porodiscida in which the margin of the central disk is armed with four chambered arms. In _Stauralastrum_ these four arms are quite simple and equal, without a patagium, separated by four right angles, so that the whole shell represents a regular rectangular cross. If we connect the distal points of the arms by lines, we get a complete square. (In my Prodromus, 1881, the species of this genus were united with _Hagiastrum_, which genus I now retain for the simple bilateral Tessarastrida.)

Subgenus 1. _Stauralastrella_, Haeckel.

_Definition._--Ends of the arms blunt, without terminal spines.

1. _Stauralastrum cruciforme_, n. sp. (Pl. 45, fig. 6).

Arms very thin, nearly linear, four to five times as long as broad, of equal breadth at the base and at the truncated distal end; their breadth equals one-third of the radius of the central disk. Edges of the arms parallel.

_Dimensions._--Radius of each arm 0.8, breadth 0.016.

_Habitat._--South Pacific, Station 293, surface.

2. _Stauralastrum lanceolatum_, n. sp.

Arms lanceolate, three times as long as broad, in their middle part three times as broad as at both ends; their greatest breadth nearly equals the diameter of the central disk. (The arms have the same form as in _Euchitonia lanceolata_, Pl. 43, fig. 9.) Edges of the arms convex.

_Dimensions._--Radius of each arm 0.3, greatest breadth (in the middle part) 0.08.

_Habitat._--Pacific, central area, Station 273, depth 2350 fathoms.

3. _Stauralastrum ordo_, n. sp.

Arms trapezoid, about as long as broad, twice as broad at their truncated distal end as at the base; their basal breadth equals the radius of the central disk, which exhibits two to three rings. (The arms have nearly the same form as those in _Hagiastrum mosis_, Pl. 45, fig. 3.) Edges of the arms rectilinear, divergent towards the ends.

_Dimensions._--Radius of each arm 0.12, basal breadth 0.04, distal breadth 0.08.

_Habitat._--Pacific, central area, Station 265, depth 2900 fathoms.

{541}4. _Stauralastrum clavigerum_, n. sp.

Arms twice as long as broad, in their distal half lenticular, nearly circular, twice as broad as in their square proximal half; their distal breadth equals the diameter of the central disk, which exhibits three to four rings. Edges of the arms concave.

_Dimensions._--Radius of each arm 0.2, basal breadth 0.04, distal breadth 0.08.

_Habitat._--Pacific, central area, Station 268, depth 2900 fathoms.

5. _Stauralastrum dilatatum_, n. sp.

Arms of nearly equal length and breadth, at their convex distal end three times as broad as at their narrow base; their distal breadth three times as large as the radius of the central disk, which exhibits three to four rings. (Resembles _Histiastrum quadrigatum_, Pl. 46, fig. 3, but has no patagium.) Edges of the arms concave.

_Dimensions._--Radius of each arm 0.15, basal breadth 0.04, terminal breadth 0.12.

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

Subgenus 2. _Stauralastromma_, Haeckel.

_Definition._--Ends of the arms with one or more terminal spines.

6. _Stauralastrum rhopalophorum_, n. sp. (Pl. 45, fig. 1).

_Hagiastrum rhopalophorum_, Haeckel, 1881, Prodromus, p. 460.

Arms cylindrical, eight times as long as broad at their base, at their distal end club-shaped, three times as broad as at their base; their distal breadth twice as large as the diameter of the central disk, which exhibits two to three rings. Surface thorny, with larger spines towards the end, and one radial, very strong, angular terminal spine. Edges of the arms parallel.

_Dimensions._--Radius of each arm (without terminal spine) 0.32, basal breadth 0.03, terminal breadth 0.1.

_Habitat._--Pacific, central area, Station 265, depth 2900 fathoms.

7. _Stauralastrum antiquum_, n. sp.

_Rhopalastrum_ sp. Bury, 1862, Polycystins of Barbados, pl. xiv. fig. 5.

Arms six times as long as broad at their base, in their distal half nearly spherical, three times as broad as in their cylindrical basal half; their distal breadth nearly equals the diameter of the central disk which exhibits three to four rings. On the end of each arm one strong, angular, terminal spine. (Differs from the preceding species by larger central disk and stouter arms, also by less developed spines.) Edges of the arms parallel.

_Dimensions._--Radius of each arm 0.25, basal breadth 0.04, terminal breadth 0.12.

_Habitat._--Fossil in the Barbados rocks; and living in the depths of the Central Pacific, Station 266, depth 2750 fathoms.

{542}8. _Stauralastrum staurolonche_, n. sp.

Arms four times as long as broad at their base, gradually increasing towards their truncated end, which is one and a half times as broad as their base; their distal breadth equals the radius of the central disk, which exhibits four to five rings. At the end of each arm is a very strong conical terminal spine. (Resembles _Histiastrum quaternarium_, _Abhandl. k. Akad. Wiss. Berlin_, 1875, Taf. xxiv. fig. 3, but has no patagium.) Edges of the arms rectilinear, divergent.

_Dimensions._--Radius of each arm 0.25, basal breadth 0.045, distal breadth 0.07.

_Habitat._--Fossil in the Barbados rocks; and living in the depth of the Equatorial Atlantic, Station 348, depth (2450) fathoms.

9. _Stauralastrum horridum_, n. sp.

Arms three times as long as broad at their base, gradually increasing towards their rounded end, which is twice as broad as their base, their distal breadth equals the diameter of the central disk, which exhibits four to five rings. Surface thorny, at the distal end of each arm is a group of twenty to twenty-five smaller and five to six larger, straight, conical spines. Edges of the arms rectilinear, divergent.

_Dimensions._--Radius of each arm 0.15, basal breadth 0.05, distal breadth 0.1.

_Habitat._--Pacific, central area, Station 270, depth 2925 fathoms.

Genus 234. _Hagiastrum_,[273] Haeckel, 1881, Prodromus, p. 460.

_Definition._--#Porodiscida# with four simple, undivided, chambered arms, without a patagium; quadrangular shell bilateral, two opposite arms of the main axis (or principal arms) different from the two others (or lateral arms).

The genus _Hagiastrum_, as here defined, was formerly united by me with the foregoing _Stauralastrum_, but differs from it by the bilateral or symmetrical form. Whilst in the latter all four arms and the four angles between them are equal, they are here differentiated into pairs.

Subgenus 1. _Hagiastrella_, Haeckel.

_Definition._--Both longitudinal arms of equal size and form.

1. _Hagiastrum buddhae_, n. sp. (Pl. 45, fig. 5).

Cross rectangular. Both longitudinal arms of equal size, twice as long as the transverse arms; all arms smooth, club-shaped, twice as broad at their globose distal part as at their base, each with three large conical terminal spines.

{543}_Dimensions._--Radius of the principal arms 0.4, of the lateral arms 0.2; basal breadth 0.06, distal breadth 0.12.

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

2. _Hagiastrum bramae_, n. sp.

Cross rectangular. Both longitudinal arms of equal size, one and a half times as long as the transverse arms; all arms thorny, club-shaped, at their pear-shaped distal part three times as broad as at their base, provided with numerous conical spines, one larger terminal spine at their distal point. (Resembles _Stauralastrum rhopalophorum_, Pl. 45, fig. 1, but is distinguished by the different size of the arm-pairs, and by the stronger spines.)

_Dimensions._--Radius of the principal arms 0.3, of the lateral arms 0.2; basal breadth 0.04, distal breadth 0.12.

_Habitat._--Indian Ocean, off Maldive Islands, surface, Haeckel.

Subgenus 2. _Hagiastromma_, Haeckel.

_Definition._--The two longitudinal arms different in size or form.

3. _Hagiastrum mosis_, n. sp. (Pl. 45, fig. 3).

Cross rectangular. All four arms nearly isosceles, triangular, at their narrow base half as broad as at their truncated, concavely fluted, distal end. The posterior principal arm with twelve to thirteen joints, twice as long as the anterior arm, which has six to seven joints and is one and a half times as long as the two lateral arms (with four to five joints).

_Dimensions._--Radius of the posterior arm 0.3, of the anterior 0.15, of the lateral arms 0.1; basal breadth 0.05, terminal breadth 0.1.

_Habitat._--Mediterranean (Smyrna), surface, Haeckel.

4. _Hagiastrum mohammedis_, n. sp.

Cross with unequal angles, the anterior little smaller than the posterior. All four arms club-shaped, thorny, three times as broad at their globose distal end as at their narrow base, and furnished with ten to twelve conical spines. Posterior principal arm twice as long as the anterior, and four times as long as the rudimentary lateral arms.

_Dimensions._--Radius of the posterior arm 0.4, of the anterior 0.2, of the lateral arms 0.1; basal breadth 0.02 to 0.03, distal breadth 0.06 to 0.1.

_Habitat._--Philippine Sea, Samboangan, Station 200, surface.

5. _Hagiastrum christi_, n. sp.

Cross with unequal angles, the anterior somewhat smaller than the posterior. All four arms of similar form, lanceolate, in their middle twice as broad as at either obtuse end. The posterior {544}principal arm with twelve joints, one and a half times as long as the anterior (with nine joints) and twice as long as the two lateral arms (each with six joints). The form and structure of the arms in this species are nearly the same as in _Tessarastrum straussi_ (Pl. 45, fig. 8); but the arms are broader in the middle, and are not connected by a patagium.

_Dimensions._--Radius of the principal posterior arm 0.2, of the anterior 0.15, of each lateral arm 0.1; greatest breadth (in the width) 0.01, basal breadth 0.03.

_Habitat._--North Atlantic, Færöe Channel, Gulf Stream, surface, John Murray.

Genus 235. _Histiastrum_,[274] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.

_Definition._--#Porodiscida# with four simple, undivided, chambered arms, connected by a patagium; square shell a regular cross, with four equal arms and four right angles between them.

The genus _Histiastrum_, quite insufficiently characterised by Ehrenberg (1847), was afterwards (1875) illustrated by the figures of two different fossil species. One of these, _Histiastrum ternarium_, with three arms, belongs to _Hymeniastrum_; the other, _Histiastrum quaternarium_, is here retained as the true, typical representative species of the genus. It differs from its ancestral form _Stauralastrum_, by the possession of a patagium, from _Tessarastrum_ by the regular square form of the shell.

Subgenus 1. _Histiastrella_, Haeckel.

_Definition._--Distal ends of the arms blunt, without terminal spines.

1. _Histiastrum quadrigatum_, n. sp. (Pl. 46, fig. 3).

Arms at their distal end nearly as broad as long, and four times as broad as at their narrow base; their lateral edges concave, their terminal edge convex, without spines. Each arm is divided by seven to eight convex transverse septa into eight to nine simple, broad chambers. Central disk with three to four rings, about as broad as the fifth chamber. Patagium complete, connecting all the lateral edges of the arms.

_Dimensions._--Radius of each arm 0.15, basal breadth 0.03, terminal breadth 0.12.

_Habitat._--Equatorial Atlantic, Station 347, surface.

2. _Histiastrum excisum_, n. sp.

Arms four times as long as broad at their base, and twice as broad at their rounded blunt distal end as at their base; their lateral edges rectilinear, divergent. Central disk with three to four rings, {545}somewhat broader than their distal end. Patagium incomplete, connecting only the basal half of the arms, with three to four concave chamber-rows, on the margin concave. (May be regarded as _Euchitonia mülleri_, with four arms.)

_Dimensions._--Radius of each arm 0.2, basal breadth 0.05, distal breadth 0.1.

_Habitat._--Atlantic, surface, Canary Islands.

3. _Histiastrum velatum_, n. sp. (Pl. 46, fig. 4).

_Dictyastrum velatum_, Haeckel, 1879, Atlas (pl. xlvi. fig. 4).

Arms pear-shaped, rapidly increasing from their narrow base, nearly circular, little longer than broad; each with eight to nine transverse chamber-rows; their lateral edges at their base concave, at their end circular. Central disk with three to four rings, somewhat smaller than one arm. Patagium complete, with six to seven radial beams, filling out perfectly the intervals between the arms. A peculiar girdle of finer network and equal breadth surrounds the whole equatorial periphery of the disk, and gives it the appearance of a square with rounded corners.

_Dimensions._--Radius of each arm 0.2, basal breadth 0.03, distal breadth 0.12.

_Habitat._--South Atlantic, Station 330, surface.

Subgenus 2. _Histiastromma_, Haeckel.

_Definition._--Distal ends of the arms spiny, furnished with one or more terminal spines.

4. _Histiastrum quaternarium_, Ehrenberg.

_Histiastrum quaternarium_, Ehrenberg, 1875, Abhandl. d. k. Akad. d.
Wiss. Berlin, p. 74, Taf. xxiv. figs. 3, 4.

Arms six times as long as broad at their base, with rectilinear, little divergent edges; at their truncated distal end a little broader, with one single, very strong, conical, terminal spine. Central disk with four to five rings, somewhat broader than the arm. Patagium incomplete, enveloping only the basal half of the arms.

_Dimensions._--Radius of each arm 0.18, basal breadth 0.03, distal breadth 0.05.

_Habitat._--Fossil in the rocks of Barbados.

5. _Histiastrum gladiatum_, n. sp.

_Astromma_ sp., Bury, 1862, Polycystins of Barbados, pl. v. fig. 1.

Arms triangular, eight times as long as broad at their base; at their distal end three times as broad as at their base, with rectilinear, divergent edges; their truncated end with a large conical, terminal spine. Central disk with four to five rings, broader than the arms. Patagium incomplete, enveloping only the basal half of the arms.

{546}_Dimensions._--Radius of each arm 0.12, basal breadth 0.013, distal breadth 0.04.

_Habitat._--Fossil in the rocks of Barbados; and living in the depth of the Central Pacific, Station 268, depth 2900 fathoms.