Definition.—Internal cavity of the shell without axial rods.
2. Stylodiscus amphistylus, n. sp.
Disk with smooth surface and smooth margin. Pores regular, circular; eight to nine on the radius of the disk. Both opposite polar spines pyramidal, sulcated, about as long as the radius of the disk, and three times as long as broad at the base. Inner cavity of the disk simple, without centripetal rods.
Dimensions.—Diameter of the disk 0.15, of the pores 0.007; length of the polar spines 0.08, basal breadth 0.03.
Habitat.—Pacific, central area, Station 274, depth 2750 fathoms.
Genus 178. Theodiscus,[[217]] n. gen.
Definition.—Cenodiscida with three radial spines on the margin of the disk, placed in its equatorial plane.
The genus Theodiscus is the most simple form of those very numerous Discoidea in which three rays are developed on the margin of the disk; a symbol of the Christian dogma of the Trinity. Commonly the three angles between the three spines are equal, more rarely one angle is larger than the two others which are equal. The shell of some species is nearly spherical (formerly separated by me as Theosphæra), whilst in others it is a flattened biconvex lens.
Subgenus 1. Theodiscoma, Haeckel.
Definition.—Angles between the three radial spines equal (triangle equilateral).