1010 THE VOYAGE OF H.M.S. CHALLENGER. 



2. Prismatium tripleurum, Haeckel. 



Prismatium tripleurum, Haeckel, 1862, Monogr. d. Radiol., p. 270, Taf. iv. fig. 6. 

 Acanthodesmia prismatium, Haeckel, I860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, 

 p. 799. 



Mitral and basal rings of equal size, equilateral triangular, both connected at the corners by 

 three vertical equal cohimellas. From the six corners arise six short, equal, conical radial spines. 

 The nine rods of the shell correspond to the nine edges of a triangular equilateral prism. 



Dimensions. Length of the shell O044, breadth 0'032. 



Habitat. Mediterranean (Messina), surface. 



Genus 437. Pseudocubus, 1 n. gen. 



Definition. T ympanida with two simple horizontal rings of different sizes, 

 connected by four divergent columellse. 



The genus Pseudociibus exhibits the characteristic form of an obelisk, or a truncated 

 four-sided pyramid. The four edges of this obelisk, or the four " columellse," are the 

 remaining parts of the two incomplete vertical rings ; two opposite edges are the dorsal 

 and ventral rod of the sagittal ring ; the two alternate edges between these are the 

 lateral rods of the frontal ring. The two horizontal rings, which become connected by 

 the four divergent columellae, are the smaller mitral ring and the larger basal ring ; 

 both are either square or nearly circular. 



1. Pseudociibus obelisciis, n. sp. (PI. 94, fig. 11). . 



Mitral ring square', with circular gate and two divergent simple spines on each corner. Basal 

 ring twice as broad, square, with four convex sides and one simple spine on each corner. Four 

 columellffi straight. All twelve rods with three smooth edges. 



Dimensions. Breadth of the mitral ring 0'02, of the basal ring 0'05. 



Habitat. Central Pacific, Station 266, depth 2750 fathoms. 



2. Pseudocubus octostylus, n. sp. 



Mitral ring square, with one stout pyramidal spine on each corner. Basal ring twice as broad, 

 square, also with a single strong spine on each corner. Four columellce straight. All twelve rods 

 with three smooth edges. 



Dimensions. Breadth of the mitral ring 0'04, of the basal ring O'OS. 



Habitat. Central Pacific, Station 271, depth 2425 fathoms. 



1 Pseudocubus = False cube ; i^ivoos, xvfrs. 



