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

Synopsis of the Genera of the Cenodiscida.

I. Subfamily Zonodiscida. Margin of the disk without radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Margin quite simple, without equatorial girdle,	175. Cenodiscus.
		Margin surrounded by a solid siliceous equatorial girdle,	176. Zonodiscus.
II. Subfamily Trochodiscida. Margin of the disk armed with radial spines (lying in the equatorial plane).	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Two spines opposite in one equatorial axis,	177. Stylodiscus.
		Three spines on the margin of the disk,	178. Theodiscus.
		Four spines opposite in pairs in two perpendicularly crossed equatorial axes,	179. Crucidiscus.
		Ten to twenty or more radial spines (variable in number and commonly irregular in disposition),	180. Trochodiscus.

Subfamily 1. Zonodiscida, Haeckel.

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

Genus 175. Cenodiscus,^[1] n. gen.

Definition.—Cenodiscida 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 Discoidea, 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.

1. Cenodiscus = Hollow disk; κενός, δίσκος.