Page:Scientific results HMS Challenger vol 18 part 1.djvu/619

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Definition.— without radial spines on the margin of the disk.

Definition.— with simple margin of the circular disk, without surrounding equatorial girdle and without radial spines.

The genus Cenodiscus is the most simple and primitive form of all, and represents possibly the common ancestral form of this order. The latticed shell is a simple biconvex lens, and encloses a smaller central capsule of the same form, separated from it by the jelly-veil. Cenodiscus can be derived phylogenetically either from Cenosphæra by lenticular flattening of a simple latticed sphere, or directly from Actissa by formation of a lenticular fenestrated shell around the lentiform central capsule. Possibly also some forms of Cenodiscus can be derived from Sethodiscus by reduction and loss of the medullary shell.

1. Cenodiscus phacoides, n. sp. (Pl. 48, figs. 1, 1a).

Disk with smooth surface, without radial ribs or spines. Margin of the lenticular biconvex disk thin, simple. Pores regular, circular; fifteen to sixteen on the radius of the disk. (Very similar to Sethodiscus phacoides, but without medullary shell.)

Dimensions.—Diameter of the disk 0.2, of the pores 0.005.

Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.