Page:Scientific results HMS Challenger vol 18 part 2.djvu/492

1368 of half the breadth, with larger hexagonal meshes. Thorax and abdomen pierced by four stout radial ribs, lying opposite in pairs in two diameters, perpendicular one to another. No secondary or interradial ribs are interpolated between the four primary. Mouth truncate, circular.

Dimensions.—Length of the three joints, a 0.02, b 0.08, c 0.04; breadth, a 0.05, b 0.18, c 0.25.

Habitat.—Central Pacific, Station 274, surface.

4. Theophormis senaria, n. sp.

Shell flat, hat-shaped, similar to Theophormis callipilium (Pl. 70, figs. 1-3), but with a different number and arrangement of the piercing radial ribs. The cephalis and its collar septum exhibit the same formation as in Sethophormis hexalactis, described above (p. 1245, Pl. 56, fig. 6.) The caudal and sternal ribs, which are opposed in the sagittal plane, are simple, whilst the two lateral ribs are forked. In the flat abdomen there are six secondary or interradial ribs, interpolated between the six primary or perradial ribs of the thorax. Delicate network with regular, hexagonal meshes.

Dimensions.—Length of the three joints, a 0.035, b 0.075, c 0.025; breadth, a 0.07, b 0.2, c 0.3.

Habitat.—Central Pacific, Station 272, surface.

Definition.— (vel Tricyrtida multiradiata aperta) with numerous radial ribs enclosed in the wall of the thorax, and of the ovate or cylindrical abdomen; mouth of the latter constricted.

The genus Phormocyrtis differs from the preceding Theophormis in the ovate or cylindrical form of the abdomen, the mouth of which is more or less constricted. The former exhibits, therefore, the same relation to the latter that Sethamphora bears to Sethophormis among the Dicyrtida. Phormocyrtis may be derived directly either from Sethamphora by development of an abdomen, or from Pterocorys or Theopilium by multiplication of the three radial ribs.

1. Phormocyrtis carinata, n. sp.

Shell ovate, with two slight strictures. Length of the three joints = 1 : 2 : 3, breadth = 1 : 3 : 4. Cephalis hemispherical, with a stout pyramidal horn of twice the length, and few very small pores. Pores in the campanulate thorax and the inflated abdomen regular, circular, small, and numerous. Six stout prominent ribs, beginning from the collar stricture, arise along the thorax and abdomen, and reach the constricted mouth, which is half as broad as the abdomen. (This species is very similar to Pterocorys carinata, from which it differs in the six complete radial ribs of the shell-wall.)

Dimensions.—Length of the three joints, a 0.03, b 0.06, c 0.09; breadth, a 0.03, b 0.09, c 0.12.

Habitat.—Tropical Atlantic, Station 347, depth 2250 fathoms.