Genus 429. Octotympanum,[[48]] n. gen.
Definition.—Tympanida with two bisected horizontal rings converging laterally and connected in the lateral poles of the equatorial plane. Eight large gates.
The genus Octotympanum differs from all other Tympanida in the peculiar connection of the two horizontal rings, which here reach the maximum of their growth, converge towards the equator, and come into direct contact on the poles of the equatorial transverse axis. Therefore the shell appears to be composed of three complete vertical meridian rings, the middle of which (the sagittal ring) is free and simple; the two other (subfrontal) rings are grown together at the lateral poles of the equatorial plane; commonly at these poles there arises (on the right and left) a strong lateral spine, and the base of this (or the crossing point of the two touching horizontal rings) corresponds to the remnants of the shortened frontal ring. The crown-shaped shell exhibits therefore eight large gates; two superior (mitral) and two inferior basal gates, two larger anterior (facial) and two larger posterior (occipital) gates. Seen from the lateral poles only two of the former are visible, and represent the figure of a vertical 8; seen from the sagittal poles, only two of the latter are visible, in the form of a larger horizontal
. Octotympanum may be derived from Acrocubus by reduction of the lateral frontal ring to a knot (or cross-point), and by maximal extension of the two parallel horizontal rings, which become curved downwards laterally and touch at the lateral poles.
1. Octotympanum octospinum, n. sp.
Mitral gates smaller than the basal gates; upper halves of the two lateral meridional rings smaller than the lower halves. Rings with few scattered thorns; four larger simple spines, nearly horizontal in the equatorial plane; two on the poles of the sagittal axis, two on the poles of the lateral axis. Four other large spines, nearly vertical at the four lowermost basal points of the subfrontal rings.
Dimensions.—Length of the shell 0.09, breadth 0.12.
Habitat.—Central Pacific, Station 273, depth 2350 fathoms.
2. Octotympanum octonarium, n. sp. (Pl. [94], fig. 3).