Page:Scientific results HMS Challenger vol 18 part 1.djvu/260

52 Regarding the number of the concentric shells which compose the latticed carapace of the, we can distinguish six families, viz.:—

I. Monosphærida (with one single shell). II. Dyosphærida (with two concentric shells). III. Triosphærida (with three concentric shells). IV. Tetrasphærida (with four concentric shells). V. Polysphærida (with five or more concentric shells). VI. Spongosphærida (with spongy shells).

On the other hand, regarding the number of the radial spines and their regular disposition on the shell-surface, we can distinguish five families, viz.:—

I. Liosphærida (without radial spines). II. Stylosphærida (with two radial spines, opposite in one axis). III. Staurosphærida (with four radial spines, opposite in pairs in two axes, perpendicular one to another). IV. Cubosphærida (with six radial spines, opposite in pairs in the three dimensive axes). V. Astrosphærida (with numerous—eight, twelve, twenty, or more—radial spines, often more than a hundred).

All five latter groups contain representatives of all six former groups; therefore we get together not less than thirty different subfamilies of, already enumerated in my Prodromus, 1881, p. 449. I repeat them here to give a better survey of the system there employed.