Page:Scientific results HMS Challenger vol 18 part 2.djvu/283

Rh

Definition.— with the basal mouth of the shell open (vel Monocyrtida multiradiata aperta).

Definition.— (vel Monocyrtida multiradiata aperta) with simple, slender pyramidal shell and numerous radial beams (six to nine or more). Network simple, with large, unfenestrated, quadrangular meshes.

The genus Bathropyramis and the following four nearly allied genera form together a peculiar group of Archiphormida, the Litharachnida, possessing near relations to Sethopyramis and the allied Sethophormida, from which they are probably derived by reduction and loss of the small cephalis. The shell in all these forms is distinctly pyramidal, composed of numerous divergent radial beams (commonly six to nine), which form the edges of the pyramid, and are connected by parallel horizontal rings. The quadrangular meshes so produced are quite simple in Bathropyramis. This genus cannot be sharply separated from Sethopyramis, its ancestral form.

Definition.—Shell smooth, without spines on the surface.

1. Bathropyramis acephala, n. sp.

Shell smooth, slenderly pyramidal, with three cortinar pores at the apex and with six stout radial beams or longitudinal ribs, connected by twelve to sixteen complete hexagonal horizontal rings. Meshes nearly square, increasing gradually in size towards the mouth.

Dimensions.—Shell 0.25 long, 0.15 broad.

Habitat.—South Pacific, Station 295, depth 1500 fathoms.

2. Bathropyramis quadrata, n. sp. (Pl. 54, fig. 1).

Shell smooth, slenderly pyramidal, with three cortinar pores at the apex and nine stout radial beams, connected by eight to twelve complete circular horizontal rings. Meshes subregular, square, increasing in size gradually towards the circular mouth.

Dimensions.—Shell 0.2 to 0.3 long, 0.12 to 0.2 broad.

Habitat.—Central Pacific, Station 272, depth 2600 fathoms.