Page:Scientific results HMS Challenger vol 18 part 2.djvu/209

Rh

Definition.— with three basal feet (an odd caudal and two paired pectoral).

Definition.— with three basal feet and one apical horn.

The genus Acrospyris is probably the oldest and most primitive among the Phormospyrida, perhaps the original ancestral form of this family. It has been derived from Tripospyris by the development of lattice-work between the three basal feet, forming a second joint or thorax. It has also therefore a close relation to the simplest forms of the triradiate Dicyrtida: Dictyophimus, Clathrocanium, &c. It differs from these in the distinct sagittal constriction of the cephalis, which divides it into two lateral loculi.

1. Acrospyris clathrocanium, n. sp. (Pl. 95, fig. 17).

Shell tuberculate, with distinct collar and deep sagittal stricture. Cephalis nut-shaped, with numerous irregular, roundish pores; three pairs of larger annular pores on each side of the ring. Basal plate with four larger pores (two major cardinal and two minor jugular pores). Apical horn slender, pyramidal, curved, about as long as the shell, with three recurved short spines. Thorax three-sided, about twice as large as the cephalis, with three large basal holes and a broad riband with numerous smaller holes; three ribs between them stout, pyramidal, strongly curved, prolonged into three convergent crescentic feet.

Dimensions.—Cephalis 0.08 long, 0.11 broad; thorax 0.14 long, 0.16 broad.

Habitat.—Central Pacific, Station 271, depth 2425 fathoms.

2. Acrospyris dictyophimus, n. sp.

Shell smooth. Cephalis and thorax of nearly the same structure and form as in the preceding species; it differs from the latter in the shape of the three stout basal feet, which are