2. Spongoxiphus prunococcus, n. sp. (Pl. 17, figs. 12, 13).
Cortical shell one and a third times as long as broad, with smooth surface. Spongy framework very compact, with very small meshes, its thickness about equals the breadth of the outer medullary shell. Both medullary shells ellipsoidal, the outer three times as large as the inner. Polar spines very stout, three-sided pyramidal, about half as long as the cortical shell, on the base about as broad as the outer medullary shell. (Differs from the preceding in the ellipsoidal form of both medullary shells, and in the finer structure of the spongy framework.)
Dimensions.—Length of the cortical shell 0.18 to 0.2, breadth 0.14 to 0.16; thickness of the spongy wall 0.035 to 0.04; length of the outer medullary shell 0.04 to 0.06, breadth 0.03 to 0.04.
Habitat.—Pacific, central area, Stations 265 to 268, depths 2700 to 2900 fathoms.
Artiscida, Haeckel, 1881, Prodromus, p. 462.
Definition.—Prunoidea with an ellipsoidal twin-shell divided by an equatorial stricture into two communicating hemiellipsoidal or hemispherical chambers, without enclosed medullary shell. Central capsule ellipsoidal, with or without equatorial stricture.
The family Artiscida has a simple fenestrated outer shell, like that of the Ellipsida, but differs from these in the presence of an equatorial constriction, by which it assumes a characteristic twin form, somewhat similar to a violin (Pl. 39, figs. 9, 10). From the Cyphinida, which have the same form of the cortical shell, the Artiscida differ in the absence of the medullary shell. It is possible that the Artiscida are descended from the Cyphinida (by loss of the medullary shell), but it is more probable that they arise from the Ellipsida by an annular constriction in the equatorial plane (perhaps the formation of the shell originally took place while the central capsule was undergoing division). Both halves of the twin shell are always of the same size and form. Its outer surface is either smooth or covered with radial spines (Artiscus, Pl. 39, fig. 9). Sometimes on the opposite poles of the main axis are developed solid spines (Stylartus, Pl. 48, fig. 5) or hollow fenestrated tubes (Cannartus, Pl. 39, fig. 10).
The Central Capsule is either simply ellipsoidal, or has also an equatorial constriction, which divides it into two equal halves. It is constantly smaller than the surrounding shell, and separated from its inner surface by a thicker or thinner jelly-mantle, the calymma.
Synopsis of the Genera of Artiscida.
| On the poles of the main axis neither solid spines nor hollow tubes, | 152. Artiscus. |
| On the poles of the main axis two solid spines (or bunches of spines), | 153. Stylartus. |
| On the poles of the main axis two hollow fenestrated tubes, | 154. Cannartus. |
