Cortical shell octangular, one and a fourth times as long as broad, with eight strong conical spines on the eight corners; these are separated by four deep constrictions on the poles of the principal and transverse axes, and by four truncated planes on the poles of the crossed diagonal axes between the former. Sagittal constriction twice as long as the hexagonal medullary shell. Four cupolas kidney-shaped or nearly pentagonal.
Dimensions.—Length of the cortical shell 0.15, breadth 0.12; length of the medullary shell 0.07, breadth 0.04.
Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.
4. Zonarium tetratholium, n. sp.
Cortical shell quadrangular, with four rounded corners. Surface thorny, with sixteen stronger radial spines; eight of these lie in the lateral plane, in the same symmetrical disposition as in the foregoing species; eight others lie on both sides of the lateral plane, opposite in pairs in two crossed diagonal planes, in the same symmetrical disposition as in Tetrapyle octacantha. Sagittal constriction three times as long as the lentelliptical medullary shell. Four cupolas obliquely elliptical.
Dimensions.—Length of the cortical shell 0.16, breadth 0.12; length of the medullary shell 0.04, breadth 0.03.
Habitat.—North Pacific, Station 241, depth 2300 fathoms.
Genus 302. Zoniscus,[[342]] n. gen.
Definition.—Zonarida with six dome-shaped chambers of the cortical shell, separated by three annular constrictions (one sagittal and two transverse, parallel to the equatorial plane).
The genus Zoniscus differs from Zonarium as well as from Zonidium by the development of the transverse girdle, which is not constricted, but on the contrary prominently vaulted in the equatorial plane. Therefore both wings of the transverse girdle form here two opposite lateral or "equatorial cupolas," as in Amphitholus. These are separated from four other domes (the "corner cupolas") by two transverse annular constrictions, which correspond to the free edges of the original transverse girdle. The corner domes of each pair are separated from each other by the sagittal septum.