Family XVII. Zygartida, Haeckel (Pl. , figs. 10-13)

Zygartida, Haeckel, 1881, Prodromus, p.462 (sensu restricto).

Definition.—Prunoidea with annulated cortical shell, the external shell being divided by five or more parallel transverse constrictions into six or more chambers, enclosing in the centre two internal concentric shells (medullary shells). Central capsule cylindrical, commonly annulated (corresponding to the transverse annular constrictions of the cortical shell).

The family Zygartida, the seventh and last of the Prunoidea, comprises, in the sense here restricted, all those Prunoidea in which the cortical shell is annulated and composed of six to twenty or more (at least six) cameræ, lying one behind another in the elongated main axis, and separated by five or more annular constrictions. When I constituted this family in 1881 (loc. cit.), I had given to it a much wider extent, embracing all those Prunoidea which exhibit annular constrictions of the cortical shell; as the number of these, in consequence of further researches, is much increased, it seems now more convenient to restrict the family to the extent here given.

No doubt the Zygartida must be derived from the Panartida by progressive growth of the cortical shell in the main axis and accompanying increase of the number of its chambers. Whilst this number in the Panartida is constantly restricted to four, in the Zygartida it amounts to ten, twenty, or more, and is in the lowest case six (Pl. , fig. 10). The maximum number is variable in the different species, but of course also different in the various degrees of individual development. Each Zygartid is at the beginning of its growth a Cyphinid, later a Panartid. The number of the annular constrictions separating the chambers and lying in parallel transverse planes is at least five, but may amount to nineteen, to twenty-one or more; they lie constantly in pairs on both sides of the equatorial constriction (Pl. , figs. 12, 13).

The cortical shell may either remain simple (Ommatocampe, Pl. , fig. 10), or become double (Desmocampe, Pl. , fig. 12), or sometimes triple (Zygocampe, (Pl. , fig. 13). In the latter cases the outer (secondary and tertiary) cortical shells are commonly incomplete, and only developed around the proximal chambers of the complete first (primary) cortical shell, its distal chambers remaining simple.

The Medullary Shell is constantly double, as in the Panartida; its form is either spherical or lenticular, compressed in the direction of the main axis. It is always connected with the equatorial constriction of the cortical shell by a number of radial beams, lying either in the equatorial plane or on each side of it (Pl. , figs. 10-13).

The Central Capsule of the Zygartida is constantly cylindrical; its increasing growth on both poles of the axis corresponds to that of the including cortical shell. Commonly (but not always) its cylindrical surface is annulated, with five or more transverse strictures, corresponding to those of the cortical shell. From the inner surface of the latter it is separated by a jelly-mantle, the calymma.

Synopsis of the Genera of Zygartida.

III. Ommacampida.
1. Cortical shell simple.
(Medullary shell double.) Without polar tubes, 169. Ommatocampe.

With two hollow fenestrated tubes, on the poles of the axis, 170. Ommatartus.

III. Desmocampida.
2. Cortical shell double.
(Medullary shell double.) Without polar tubes, 171. Desmocampe.

With two hollow fenestrated tubes, on the poles of the axis, 172. Desmartus.

III. Zygocampida.
3. Cortical shell triple.
(Medullary shell double.) Without polar tubes, 173. Zygocampe.

With two hollow fenestrated tubes, on the poles of the axis, 174. Zygartus.

I. Ommacampida. 1. Cortical shell simple. (Medullary shell double.)

Without polar tubes,

169. Ommatocampe.

With two hollow fenestrated tubes on the poles of the axis,

170. Ommatartus.

II. Desmocampida. 2. Cortical shell double. (Medullary shell double.)

Without polar tubes,

171. Desmocampe.

With two hollow fenestrated tubes on the poles of the axis,

172. Desmartus.

III. Zygocampida. 3. Cortical shell triple. (Medullary shell double.)

Without polar tubes,

173. Zygocampe.

With two hollow fenestrated tubes on the poles of the axis,

174. Zygartus.

Genus 169. Ommatocampe, Ehrenberg, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 832.

Definition.—Zygartida with simple cortical shell and double medullary shell, without polar tubes.

The genus Ommatocampe was founded by Ehrenberg in 1860 for one of his "Haliommatina," with the following diagnosis:—"Shell rod-like, long, articulate, with nucleus, without spines, with four or more joints." The species figured by him, Ommatocampe polyarthra, exhibits five pairs of chambers on both sides of the meridian plane of the cortical shell, the centre of which includes a double medullary shell. It represents the most simple form of all Zygartida, and may be derived phylogenetically from Cyphonium simply by multiplication of the chambers of the cortical shell, growing on both poles of the main axis.

Subgenus 1. Ommatocampium.

Definition.—Surface of the cortical shell smooth or rough, without thorns or spines, also without polar spines.

1. Ommatocampe polyarthra, Ehrenberg.

Ommatocampe polyarthra, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 279, Taf. vi. fig. 9.

Cortical shell with smooth surface, composed of six to ten chambers of the same size and form. Every chamber kidney-shaped, about twice as broad as long, with three transverse rows of circular, subregular pores, twice to three times as broad as the bars. Both medullary shells spherical.

Dimensions.—Length of the six-chambered cortical shell 0.15; greatest breadth of each chamber 0.04; pores 0.005 to 0.007, bars 0.002 to 0.003.

Habitat.—Californian Sea, depth 2600 fathoms, Ehrenberg; Pacific, central area, Station 268, depth 2900 fathoms; fossil in the Tertiary rocks of Barbados, Haeckel.

2. Ommatocampe increscens, Stöhr.

Ommatocampe increscens, Stöhr, 1880, Palæontogr., vol. xxvi. p. 90, Taf. ii. fig. 2, a, b.

Cortical shell with rough surface, composed of six to ten chambers of nearly the same size and form; the breadth of the chambers a little increasing towards both poles. Every chamber kidney-shaped, the proximal chambers twice as broad as long, with four or five transverse rows of pores; the distal chambers three to four times as broad as long, with two or three transverse rows of pores. Form of the pores irregular, roundish, bars between them in the distal part smaller, in the proximal part larger than the pores. Both medullary shells compressed lenticular.

Dimensions.—Length of the six-chambered cortical shell 0.22; greatest breadth of the chambers 0.09 to 0.1; pores and bars 0.003 to 0.006.

Habitat.—Fossil in the Tertiary rocks of Sicily, Grotte, Stöhr.

3. Ommatocampe annulata, n. sp.

Cortical shell with smooth surface, composed of six to twelve chambers of nearly the same size and form. Each chamber kidney-shaped, twice as broad as long, with four to five transverse rows of irregular, roundish pores, once to five times as broad as the bars. On the base of each distal chamber (beginning in the second or third pair) a circle of twelve to sixteen larger square pores. Both medullary shells compressed lenticular. (Resembles the inner cortical shell of Desmartus larvalis, Pl. , fig. 12, but is without the polar tubes and the outer envelope of this species.)

Dimensions.—Length of the six-chambered cortical shell 0.23; greatest breadth of each chamber 0.07; pores 0.002 to 0.01, bars 0.002.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, in various depths.

Subgenus 2. Ommatocampula, Haeckel.

Definition.—Surface of the cortical shell thorny or spiny, but without peculiar polar spines and without regular coronals of spines.

4. Ommatocampe erucæformis, n. sp.

Cortical shell with spiny surface, composed of six to twelve chambers of nearly the same size and form, the distal chambers a little smaller. The form and structure of the cortical shell is nearly the same as in the foregoing species, only the pores are somewhat larger, and the whole surface is covered with bristle-like radial spines, about half as long as the breadth of one chamber. Both medullary shells compressed and lenticular.

Dimensions.—Length of the six-chambered cortical shell 0.24; greatest breadth of each chamber 0.08; pores 0.005 to 0.012, bars 0.002.

Habitat.—North Atlantic, Canary Islands, Station 353, depth 2965 fathoms.

5. Ommatocampe nereis, n. sp. (Pl. , fig. 10).

Cortical shell with spiny surface, composed of six (or more) chambers of different size and structure. Both proximal chambers kidney-shaped, with subregular, circular, hexagonally framed pores, twice as broad as the bars. All following chambers cap-like, with much smaller, irregular, roundish pores, at the base of every chamber a circle of ten to twelve large square pores. Spines of the surface short, irregularly scattered. Both medullary shells spherical. (All the observed specimens possessed only six chambers.)

Dimensions.—Length of the six-chambered cortical shell 0.25; greatest breadth of each chamber 0.08; pores of both proximal chambers 0.01, bars 0.005; basal pores of the other chambers 0.02; pores of their distal caps 0.003 to 0.006, bars 0.003.

Habitat.—Pacific, central area, Stations 271 to 274, depth 2350 to 2750 fathoms.

Subgenus 3. Ommatocorona, Haeckel.

Definition.—Surface of the cortical shell spiny, on every chamber a regular circle or coronal of radial spines.

6. Ommatocampe chætopodum, n. sp.

Cortical shell with spiny surface, composed of six (or more) kidney-shaped chambers of different size and form. Both proximal chambers kidney-shaped, with subregular, circular, hexagonally framed pores, about as broad as the bars. All the following chambers hemispherical, with irregular, roundish pores; at the base of every chamber a circle of ten to twelve larger square pores. The beams between these latter are prolonged into free radial spines, twice as thick as the bars. Therefore every chamber is surrounded by a circle of radial spines, like those of Panicium coronatum (Pl. , fig. 4). Both medullary shells lenticular.

Dimensions.—Length of the six-chambered cortical shell 0.3, greatest breadth 0.08, pores and bars of both proximal chambers 0.01; pores of the other chambers 0.002 to 0.008; square pores 0.02, bars 0.005; length of the coronal spines 0.03, thickness 0.01.

Habitat.—Indian Ocean, Madagascar, Rabbe.

Subgenus 4. Ommatacantha, Haeckel.

Definition.—Surface of the shell smooth or spiny, on the poles of the main axis occur two strong opposite polar spines.

7. Ommatocampe amphilonche, n. sp.

Cortical shell composed of six kidney-shaped chambers of nearly the same size and structure; every chamber twice as broad as long, with four to five transverse rows of irregular, roundish pores, once to four times as broad as the bars. Both medullary shells lenticular. Surface of the cortical shell covered with bristle-like spines. On both poles of the main axis one larger, strong, conical spine, about half as long as this axis, and on the base as broad as the inner medullary shell. (Resembles on the whole Desmartus larvalis, Pl. , fig. 12, but without external mantle, and with two solid polar spines instead of the polar tubes.)

Dimensions.—Length of the six-chambered cortical shell 0.24; greatest breadth of each chamber 0.08; pores 0.002 to 0.08, bars 0.002; length of the polar spines 0.12, basal thickness 0.02.

Habitat.—Pacific, central area, Station 266, depth 2750 fathoms.

Genus 170. Ommatartus, Haeckel, 1881, Prodromus, p. 463.

Definition.—Zygartida with simple cortical shell and double medullary shell, with two hollow fenestrated tubes, opposite on both poles of the main axis.

The genus Ommatartus differs from Ommatocampe by the development of two hollow fenestrated tubes on both poles of the main axis, and bears therefore the same relation to it as Cannartidium to Cyphonium. The former two genera differ from the two latter by the augmentation of the chambers of the cortical shell.

1. Ommatartus amphicanna, n. sp.

Cortical shell with spiny surface, composed of six chambers of equal size and kidney-shaped; every chamber with four to five transverse rows of irregular, roundish pores, twice to three times as broad as the bars. Both medullary shells lenticular. Polar tubes conical, about as long as one chamber, with smaller pores. Both medullary shells spheroidal, more or less compressed. (Resembles Desmartus larvalis, Pl. , fig. 12, but is without the external shell.)

Dimensions.—Length of the six-chambered cortical shell 0.24; greatest breadth of each chamber 0.07; pores 0.008 to 0.012, bars 0.004; length of the polar tubes 0.04 to 0.05, basal thickness 0.02.

Habitat.—Pacific, central area, Stations 270 to 274, depth 2350 to 2925 fathoms.

2. Ommatartus amphisiphon, n. sp.

Cortical shell with spiny surface, composed of six or eight chambers of different size and form. Both proximal chambers kidney-shaped, each with five to six transverse rows of irregular, polygonal pores, three to four times as broad as the bars. The other (four to six) chambers somewhat smaller, more hemispherical, with smaller, irregular pores, but on the base of each a circle of ten to twelve larger square pores. Polar tubuli prismatic, about half as long as the main axis, with prominent edges and longitudinal rows of smaller pores between them. Both medullary shells lenticular. (Resembles closely Panarium tubularium, Pl. , fig. 9, but with six to eight chambers instead of four, and with longer tubuli.)

Dimensions.—Length of the six-chambered cortical shell 0.26; greatest breadth of the proximal chambers 0.09, of the distal chambers 0.07; pores of the former 0.01, of the latter 0.004; large square pores 0.013, bars 0.003; length of the polar tubes 0.1 to 0.12, thickness 0.02.

Habitat.—South Pacific, Station 297, depth 1775 fathoms.

3. Ommatartus amphobolus, n. sp.

Cortical shell with spiny surface, composed of six chambers of different size and form. Both proximal chambers kidney-shaped, with four to five transverse rows of subregular, circular pores, twice as broad as the bars. Both middle chambers cap-like, with a basal circle of ten to twelve larger square pores, and small roundish pores on the cap. Both distal chambers smaller, conical, with very small roundish pores. Polar tubuli conical, nearly half as long as the main axis, also with very small pores. Both medullary shells lenticular.

Dimensions.—Length of the six-chambered shell 0.28, greatest breadth 0.08; pores of the proximal chambers 0.01, bars 0.005; pores of the distal chambers and the polar tubes 0.002 to 0.004, bars 0.002; length of the polar tubes 0.13, basal thickness 0.03.

Habitat.—North Pacific, Station 253, depth 3125 fathoms.

Genus 171. Desmocampe, n. gen.

Definition.—Zygartida with double cortical shell and double medullary shell, without polar tubes.

The genus Desmocampe differs from Ommatocampe in the duplication of the jointed cortical shell; the radial spines, which start from the surface of the inner cortical shell, are connected one with another by transverse communicating branches which form an outer envelop around it; but this reticulated mantle is commonly not quite perfect and more or less irregular.

1. Desmocampe catenula, n. sp.

Inner cortical shell with six to eight chambers of the same size and form. Every chamber kidney-shaped, with three to four transverse rows of circular, subregular pores, twice as broad as the bars. Outer cortical shell cylindrical, hemispherical at both poles, with smooth surface and irregular, roundish pores of very different size. Distance between the two cortical shells equals the diameter of the outer medullary shell, which, like the inner, is spherical. (Resembles Ommatocampe polyarthra, Ehrenberg, 1872, loc. cit., Taf. vi, fig. 9, but differs in the external mantle.)

Dimensions.—Length of the six-chambered inner cortical shell 0.16, of the outer 0.21; greatest breadth of each chamber of the former 0.05, of the latter 0.09; pores of the inner 0.006, bars 0.003; pores of the outer 0.002 to 0.008, bars 0.002.

Habitat.—North Pacific, Station 237, off Japan, surface.

2. Desmocampe tænioides, n. sp.

Inner cortical shell with six to ten chambers of nearly the same size and form; the distal chambers somewhat smaller. Every chamber kidney-shaped, with four to five transverse rows of irregular roundish pores, twice to three times as broad as the bars. Outer cortical shell cylindrical, hemispherical at both poles, with spiny surface; its pores like those of the inner, but the bars between them much thinner. Distance between the two cortical shells equals the diameter of the outer medullary shell. Both medullary shells lenticular. (Resembles Desmartus larvalis, Pl. , fig. 12, but is without polar tubes.)

Dimensions.—Length of the six-chambered inner cortical shell 0.25, of the outer 0.3; greatest breadth of the former 0.07, of the latter 0.11; pores 0.005 to 0.012; bars of the inner shell 0.004, of the outer 0.001.

Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.

3. Desmocampe aphrodite, n. sp.

Inner cortical shell with six to ten chambers of different size and structure. Both proximal chambers kidney-shaped, with five to six transverse rows of subregular, circular, hexagonally-framed pores, twice as broad as the bars. All following chambers cap-like, with much smaller, irregular, roundish pores, on the base of each a circle of ten to twelve large square pores. Outer cortical shell cylindrical, on both poles hemispherical, with spiny surface and very delicate network of small polygonal pores. Both medullary shells spherical. (The inner cortical shell of this species resembles that of Ommatocampe nereis, Pl. , fig. 10; the outer that of Cyphocolpus virginis, Pl. , fig. 11.)

Dimensions.—Length of the six-chambered internal cortical shell 0.25, of the outer 0.3; greatest breadth of the former 0.08, of the latter 0.12; pores of the proximal chambers of the inner shell 0.01, bars 0.005; pores of the distal chambers 0.003 to 0.005, square pores 0.02, bars 0.003; pores of the external cortical shell 0.002 to 0.006, bars 0.001.

Habitat.—North Atlantic, Station 353, depth 2965 fathoms.

4. Desmocampe atractus, n. sp.

Inner cortical shell with six chambers of very different size and structure. Both proximal chambers kidney-shaped, with seven to eight transverse rows of subregular, roundish pores, twice to three times as broad as the bars. Both middle chambers cap-like, on the base with a circle of eight to ten very large square pores, on the distal cap with small irregular, roundish pores. Both distal chambers conical, also with small irregular, roundish pores. Outer cortical shell spindle-shaped, inflated in the equatorial zone, tapering conically towards both poles, with very delicate network of small roundish pores and thin bars. Surface covered with innumerable very small spines. Both medullary shells lenticular. (Resembles very much Peripanartus atractus, Pl. , fig. 7, but differs in the number of the chambers and their proportion to the outer mantle, which envelops spindle-like all six chambers.)

Dimensions.—Length of the six-chambered internal cortical shell 0.27, of the outer 0.32; greatest breadth of the former 0.09, of the latter 0.15; pores of the inner cortical shell (on an average)—proximal chambers 0.008, middle chambers 0.02, distal chambers 0.004, bars 0.004; pores of the outer cortical shell 0.002 to 0.004, bars 0.002.

Habitat.—North Pacific, Station 241, depth 2300 fathoms.

Genus 172. Desmartus, n. gen.

Definition.—Zygartida with double cortical shell and double medullary shell, with two hollow fenestrated tubes, opposite on both poles of the main axis.

The genus Desmartus differs from Desmocampe by the development of two hollow fenestrated tubes, opposite on both poles of the main axis, and bears therefore the same relation to it as Ommatartus to Ommatocampe. Both the former genera can be produced from the two latter by duplication of the cortical shell.

1. Desmartus larvalis, n. sp. (Pl. , fig. 12).

Zygartus larvalis, Haeckel, 1881, Prodromus et Atlas (pl. xl, fig. 12).

Inner cortical shell composed of six kidney-shaped chambers of nearly the same size and structure; every chamber twice as broad as long, with four to five transverse rows of irregular, roundish pores, twice to five times as broad as the bars; the basal pores of the distal chambers somewhat larger and more square. Outer cortical shell cylindrical, hemispherical at both poles, with irregular, polygonal pores, on an average twice as large as those of the inner, but the bars between them much thinner. Outer surface spiny. Both medullary shells lenticular. Polar tubes conical, a little longer than one internal chamber, as broad at the base as the inner medullary shell. Sometimes the tubes exhibit prominent edges (as in the lower spine of fig. 12); the pores of these are very small, and roundish.

Dimensions.—Length of the six-chambered internal cortical shell 0.23, of the external 0.3; greatest breadth of the former 0.07, of the latter 0.11; pores of the inner shell 0.004 to 0.01, of the outer 0.01 to 0.02; bars of the former 0.002, of the latter 0.001; length of the polar tubes 0.05, basal thickness 0.02.

Habitat.—North Pacific, Station 241, depth 2300 fathoms.

2. Desmartus tubulatus, n. sp.

Inner cortical shell composed of six to ten kidney-shaped chambers, tapering in size towards both poles, every chamber with six to seven transverse rows of irregular, roundish pores, twice to six times broader than the bars. Outer cortical shell spindle-shaped, in the equatorial zone inflated, tapering towards both poles, its network similar to the inner, only more delicate. Polar tubes conical, twice as long as an inner chamber, as broad at the base as the outer medullary shell. Both medullary shells lenticular.

Dimensions.—Length of the six-chambered internal cortical shell 0.25, of the external 0.32; greatest breadth of the former 0.08, of the latter 0.12; pores of the inner shell 0.002 to 0.012, of the outer 0.003 to 0.01; bars of the former 0.002, of the latter 0.001; length of the polar tubes 0.07, basal thickness 0.03.

Habitat.—North Pacific, Station 256, depth 2950 fathoms.

Genus 173. Zygocampe, n. gen.

Definition.—Zygartida with triple (or multiple) cortical shell and double medullary shell, without polar tubes.

The genus Zygocampe differs from Desmocampe and Ommatocampe by the multiplication of the cortical shell, which is composed of three or more concentric envelopes. The three mentioned genera form therefore a phylogenetic series, produced by the concentric increase on the outside of the jointed cortical shell. Commonly the second cortical shell is not as complete as the first (or innermost), and the third (or outermost) is yet more incomplete. Rarely the number of the concentric cortical shells surpasses three.

1. Zygocampe pupula, n. sp.

Inner cortical shell with six to twelve chambers of nearly the same size and form. Every chamber kidney-shaped, with four to five transverse rows of circular, subregular pores, twice as broad as the bars. Middle cortical shell cylindrical, hemispherical at both poles, with subregular circular pores somewhat smaller than those of the inner; the bars also thinner. Outer cortical shell of the same form as the middle, but with very delicate network, and quite irregular, roundish pores and very thin bars. Surface quite smooth. Both medullary shells lenticular. (May be regarded in a phylogenetic as well as an ontogenetic sense, as the further developmental form of Desmocampe catenula and Ommatocampe polyarthra.)

Dimensions.—Length of the six-chambered inner cortical shell 0.17, of the middle 0.22, of the outer 0.27; greatest breadth of the first 0.05, of the second 0.09, of the third 0.12; pores of the inner cortical shell 0.005, of the middle 0.004, of the outer 0.003 to 0.012; bars of the first 0.003, of the second 0.002, of the third 0.001.

Habitat.—Pacific, central area, Station 266, depth 2750 fathoms.

2. Zygocampe corasium, n. sp.

Inner cortical shell with six to eight chambers of different size and form. Both proximal chambers kidney-shaped, with five to six transverse rows of subregular, circular, hexagonally framed pores, twice as broad as the bars. All following chambers cap-like, the distal somewhat smaller; their pores much smaller, irregular, roundish; only at the base of each chamber a circle of ten to twelve large square pores. Middle cortical shell cylindrical, in the equatorial zone a little constricted, hemispherical at both poles, with irregular delicate network of roundish, polygonal meshes. Outer cortical shell of the same form as the middle, but with a very delicate and quite irregular network of polygonal meshes. Commonly this outer mantle is incomplete, and sometimes interwoven in a spongy manner with the middle (or also with the inner). Surface covered with many irregular, thin, bristle-like spines. Both medullary shells lenticular. (May be considered in a phylogenetic and ontogenetic sense as a further developmental stage of Desmocampe aphrodite and Ommatocampe nereis, Pl. , fig. 10.)

Dimensions.—Length of the six-chambered inner cortical shell 0.24, of the middle 0.3, of the outer 0.36; greatest breadth of the first 0.08, of the second 0.12, of the third 0.16, pores of the first (on an average) 0.005 to 0.02, of the second 0.003 to 0.015, of the third 0.01 to 0.05; bars corresponding 0.005 or 0.003 or 0.001.

Habitat.—North Pacific, Station 253, depth 3125 fathoms.

3. Zygocampe chrysalidium, n. sp. (Pl. , fig. 13).

Inner cortical shell with six to eighteen chambers of different size and form. Both proximal chambers kidney-shaped, with six to seven transverse rows of subregular, circular pores, three to four times as broad as the bars. All following chambers with more irregular pores, with a circle of ten to twelve larger square pores at the base. Middle cortical shell with smaller, irregular, roundish pores. Outer cortical shell with larger polygonal, quite irregular pores. Both outer shells appear commonly incomplete or somewhat irregularly developed, and sometimes in a spongy manner interwoven with one another. Surface covered with irregular, bristle-like spines. Both medullary shells spherical. The breadth of the chambers decreases gradually towards both poles, so that the whole shell assumes a spindle form. Sometimes on both poles is developed a short conical polar tube (in fig. 13 only on the upper pole), and so this species is transformed into Zygartus chrysalis.

Dimensions.—Length of the six-chambered inner cortical shell 0.25, of the middle 0.3, of the outer 0.35; greatest breadth of the first 0.07, of the second 0.11, of the third 0.14; pores (on an average) 0.01 (0.002 to 0.02), bars 0.002 to 0.005.

Habitat.—Pacific, central area, Stations 272 to 274, depth 2350 to 2750 fathoms.

Genus 174. Zygartus, Haeckel, 1881, Prodromus, p. 463.

Definition.—Zygartida with triple (or multiple) cortical shell and double medullary shell, with two hollow fenestrated tubes, opposite on both poles of the main axis.

The genus Zygartus differs from Zygocampe by the development of two hollow fenestrated tubes, on the two opposite poles of the main axis. It exhibits therefore to the latter the same relation that Desmartus does to Desmocampe and Ommatartus to Ommatocampe; it differs from these by the multiplication of the cortical shell, which is composed of at least three concentric envelopes.

1. Zygartus doliolum, n. sp.

Inner cortical shell composed of six (or more) kidney-shaped chambers, all nearly of the same size and structure, every chamber with four to five transverse rows of irregular, roundish pores, twice to four times as broad as the bars; the basal pores scarcely larger than the apical pores. Middle cortical shell nearly of the same structure as the inner, only the pores larger and the bars thinner. Outer cortical shell cylindrical, hemispherical at both poles, its network very delicate, with large irregular, polygonal pores, and very thin bars between them; its surface covered with thin bristle-like spines. Both medullary shells lenticular. Polar tube cylindrical, with conical apex, and with very small pores; its length equals the breadth of two internal chambers, its breadth that of the inner medullary shell. (This species appears to be a further developed form of Ommatocampe annulata and Desmartus larvalis, Pl. , fig. 12.)

Dimensions.—Length of the six-chambered inner cortical shell 0.25, of the middle 0.3, of the outer 0.35; greatest breadth of the first 0.07, of the second 0.11, of the third 0.15; pores of the inner shell 0.005 to 0.01, of the middle 0.003 to 0.007, of the outer 0.008 to 0.02; bars of the first 0.002, of the second and third 0.001; length of the polar tubes 0.15, basal thickness 0.02.

Habitat.—Pacific, central area, Station 270, depth 2925 fathoms.

2. Zygartus chrysalis, n. sp. (Pl. , fig. 13).

Inner cortical shell composed of six to twenty (commonly eight to twelve) chambers of different size and form. Both proximal chambers nearly hemispherical, with spiny surface and subregular, circular pores, three to four times as broad as the bars. All following chambers cap-like, tapering towards both poles, with more irregular, roundish pores, with a circle of ten to twelve larger square pores at the base. Middle cortical shell with smaller roundish pores. Outer cortical shell with larger polygonal, quite irregular pores. Both medullary shells lenticular. Polar tubes conical or cylindrical with conical apex, of very variable length, sometimes not longer than one internal chamber, at other times twice to four times as long (in the figured specimen not fully developed, as also a part of the shells). Differs from Zygocampe chrysalis only by the production of polar tubes.

Dimensions.—Length of the six-chambered inner cortical shell 0.25, of the middle 0.3, of the outer 0.35; greatest breadth of the first 0.07, of the second 0.11, of the third 0.14; other measures the same as in Zygocampe chrysalis; length of the tubes 0.05 to 0.12 or more.

Habitat.—Pacific, central area, Stations 270 to 274, depth 2350 to 2925 fathoms.