The twelve rods of the cubical shell are curved and armed with scattered, simple, short spines. The figure 7 of Johannes Müller (_loc. cit._) corresponds exactly to the Mediterranean form observed by me at Portofino, and is quite different from his true _Acanthodesmia vinculata_ (_loc. cit._, figs. 4-6), so that I have no doubt he did observe these two different species (compare above, p. 975). _Dimensions._--Diameter of the cube 0.07; length of the spines 0.02. _Habitat._--Mediterranean (Nice, Portofino). 4. _Lithocubus astragalus_, n. sp. (Pl. 82, fig. 12). The twelve rods of the cubical shell are stout and slightly curved, armed with numerous, simple, and irregularly branched spines. Eight larger spines, branched like a deer's antler, arise from the eight corners of the cube. _Dimensions._--Diameter of the cube 0.1 to 0.12; length of the corner spines 0.05 to 0.08. _Habitat._--Central Pacific, Station 271, depth 2425 fathoms. Genus 439. _Circotympanum_,[57] n. gen. _Definition._--#Tympanida# with two simple different horizontal rings, connected by numerous (six to eight or more) divergent columellæ. The genus _Circotympanum_ comprises those Tympanida in which two simple horizontal rings of different size (an upper smaller "mitral ring," and a lower larger "basal ring") are connected by a variable number of divergent rods or columellæ (at least six or eight). _Circotympanum_ is probably derived from _Tympaniscus_ or _Tympanidium_ by partial reduction of the sagittal ring, the upper (apical) and lower (basal) part of which is lost. The geometrical fundamental form is a truncated pyramid with six to eight or more edges. 1. _Circotympanum hexagonium_, n. sp. Mitral and basal ring hexagonal, the latter twice as broad as the former, each with a simple small spine on the six corners. Six columellæ nearly straight, as long as the radius of the basal ring. All rods with three prominent edges. _Dimensions._--Breadth of the mitral ring 0.06, of the basal ring 0.11. _Habitat._--Central Pacific, Station 268, depth 2900 fathoms. {1013}2. _Circotympanum octogonium_, n. sp. (Pl. 94, fig. 17). Mitral and basal rings octagonal, the latter one and a half times as broad as the former. Eight columellæ slightly curved, thorny. The mitral ring bears on each corner a short ascending spine, the basal ring a larger, thorny, obliquely descending spine, which has some recurved thorns on the upper edge. All rods more or less thorny. _Dimensions._--Breadth of the mitral ring 0.06, of the basal ring 0.09. _Habitat._--Central Pacific, Station 271, depth 2425 fathoms. 3. _Circotympanum decagonium_, n. sp. Mitral and basal rings circular, the latter nearly twice as broad as the former, both armed with scattered irregular thorns. Ten columellæ smooth, curved. _Dimensions._--Breadth of the mitral ring 0.07, of the basal ring 0.12. _Habitat._--North Pacific, Station 244, depth 2900 fathoms.