Genus 435. Parastephanus, Haeckel, 1881, Prodromus, p. 446.
Definition.—Tympanida with two simple horizontal rings, connected by two vertical columellæ.
The genus Parastephanus is the simplest among the Eutympanida, and exhibits only two vertical rods or columellæ, which connect the two parallel horizontal rings. One of these latter must be regarded as the mitral or superior ring, the other as the basal or inferior ring; whilst the two parallel columellæ, connecting both, are the remaining vertical parts of the sagittal ring, the upper and lower parts of which are lost. By loss of the latter Parastephanus has probably been derived from Protympanium.
1. Parastephanus circularis, n. sp.
Two horizontal rings smooth, circular, of equal size, connected by two curved smooth columellæ, which are somewhat shorter than the diameter of the rings.
Dimensions.—Diameter of the rings 0.08, length of the columellæ 0.06.
Habitat.—Central Pacific, Station 263, depth 2650 fathoms.
2. Parastephanus quadrispinus, n. sp. (Pl. 93, fig. 21).
Two horizontal rings smooth, elliptical or nearly circular, of equal size, connected by two vertical, slightly curved columellæ, which are about as long as the radius of the rings. Each ring bears two simple lateral spines, which in the mitral ring are curved downwards, in the basal ring upwards, possibly the last remains of a reduced frontal ring.
Dimensions.—Diameter of the rings 0.09, length of the columellæ 0.05.
Habitat.—North Pacific, Station 236, surface.
3. Parastephanus asymmetricus, n. sp. (Pl. 82, fig. 10).
Two horizontal rings thorny, of equal size, irregularly ovate or nearly semicircular, and of somewhat asymmetrical form. The two connecting parallel columellæ are straight, cylindrical, about as long as the diameter of the rings. The peculiar asymmetry in the form of the rings raises the possibility that the two columellæ in this species are the remaining parts of a frontal ring, and that the sagittal ring has altogether disappeared. In this case the convex half of both rings would be the ventral, the straight half the dorsal part.
Dimensions.—Diameter of the rings 0.1, length of the columellæ 0.08.
Habitat.—Central Pacific, Station 272, depth 2600 fathoms.
