Definition.—The two principal arms of different size or form.
4. Tessarastrum democriti, n. sp. (Pl. [45], fig. 7).
Histiastrum democriti, Haeckel, 1881, Prodromus et Atlas (pl. xlv. fig. 7).
Cross not rectangular; the two anterior angles smaller than the two posterior. All four arms club-shaped, twice as broad at their rounded obtuse distal end as at their base, of unequal length. Posterior principal arm one and a fourth times as long as the posterior, and one and two-thirds as long as the lateral arms. Patagium incomplete, enveloping only the basal half of the arms. I call this species after the great Greek philosopher Democritus.
Dimensions.—Radius of the posterior arm 0.3, of the anterior 0.25, of each lateral arm 0.22; basal breadth 0.05, distal breadth 0.1.
Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.
Genus 237. Stephanastrum,[[276]] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.
Definition.—Porodiscida with four simple, undivided, chambered arms, connected on the distal ends by a spongy, square or rhomboidal, patagial girdle (or a patagium with four large, interbrachial openings). Shell either regular or bilateral (with equal or unequal arms).
The genus Stephanastrum, founded (1847) by Ehrenberg for the very peculiar Stephanastrum rhombus, differs from the nearly allied foregoing genera in the imperfect development of the peculiar patagium, connecting only the distal ends of the four arms, while it is absent at their base. Two new species, different from Stephanastrum rhombus by the regular square form, were found in the Challenger collection.