Page:Scientific results HMS Challenger vol 18 part 1.djvu/625

Rh broad at their base as the radiated margin, which bears fifty to sixty triangular pointed teeth of irregular size. (Similar to Sethostaurus cruciatus, Pl. 31, fig. 5, but without internal axial rods and medullary shell.)

Dimensions.—Diameter of the disk 0.2, of the pores 0.007; length of the spines 0.1, basal breadth 0.02.

Habitat.—Pacific, central area, Station 274, depth 2750 fathoms.

Definition.— with numerous (ten to twenty or more) radial spines on the margin of the disk, situated in its equatorial plane.

The genus Trochodiscus bears on the thin margin of the latticed disk a variable number of radial spines (commonly ten to twenty, sometimes fifty to eighty or more). Their size and disposition are commonly more or less irregular. For the most part all the marginal spines lie in the equatorial plane; but sometimes part of them are crowded into two or four parallel girdles. Several species of this genus are very similar to some species of Heliodiscus, and only differ in the absence of the medullary shell, perhaps in consequence of the phylogenetic loss of it; if this be the case, the former are to be derived from the latter (compare p. 410).

Definition.—Bases of the marginal spines free, not connected by a solid equatorial girdle.

1. Trochodiscus cenophacus, n. sp.

Disk with smooth surface. Pores regular, circular; eleven to twelve on the radius. Marginal spines sixteen to twenty, conical, of irregular size and disposition, about three times as long as broad, and as long as the radius of the disk.

Dimensions.—Diameter of the disk 0.15, of the pores 0.006; length of the spines 0.06 to 0.09, basal breadth 0.02 to 0.03.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.

2. Trochodiscus medusinus, n. sp.

Disk with smooth surface. Pores regular, circular; nine to ten on the radius. Marginal spines ten to twelve, equilateral triangular, deeply sulcated, about half as long and broad as the radius of the disk. (Very similar to Heliosestrum medusinum, Pl. 34, fig. 6, but without enclosed medullary shell. Compare p. 410.)

Dimensions.—Diameter of the disk 0.2, of the pores 0.01; length of the spines 0.05, basal breadth 0.05.

Habitat.—North Pacific, Station 241, depth 2300 fathoms.