Page:Scientific results HMS Challenger vol 18 part 2.djvu/149

Rh

Definition.— tripoda, with three descending basal feet (the same as in Cortina, an odd posterior or caudal foot and two paired anterior or pectoral feet).

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

The genus Tripospyris (Pl. 84, figs. 1-7) is in the large group of probably the most original and typical form, from which, as a common ancestral form, all other genera of this polymorphous suborder may be derived. The shell is usually shaped like a nut or thorax, and exhibits two paired lateral chambers, which are separated by a distinct sagittal ring and a slighter or deeper sagittal constriction. The dorsal rod of the ring is prolonged at the upper pole into an ascending apical horn, at the lower pole into a descending caudal foot, whilst two paired pectoral or lateral feet arise from the anterior pole of the basal rod. These three divergent basal feet and the apical horn are the same four typical radial spines which we have encountered already in Plagoniscus (Plagonida), in Plectaniscus (Plectanida), in Cortina (Stephanida), and in Cortiniscus (Semantida). Compare above, p. 891. From all these, Tripospyris differs in the development of a complete bilocular lattice-shell, having the characters of the true. The numerous species of this genus may be placed in different subgenera, according to the different number of collar pores or cortinar pores in the basal plate (two, three, four, six, or more). These differences are very important, however difficult to make out; the following system therefore is a provisional one, and requires further accurate observations.

Definition.—Basal plate with two large pores only (the primary jugular pores of Semantis).

1. Tripospyris cortina, n. sp. (Pl. 95, fig. 1).

Shell ovate, smooth, about as long as broad, with slight ovate sagittal stricture, and broad primary ring in its wall. Basal plate with two large kidney-shaped collar pores only (cardinal pores). Facial and occipital plates each with two pairs of large annular pores, and ten to twelve pairs of small lateral pores. Apical horn and the three basal feet of equal length, one and a half times as long as the shell, three-sided prismatic, straight, divergent.

Dimensions.—Shell 0.08 long, 0.09 broad; horn and feet 0.15 long.

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