REPORT ON THE RADIOL ARIA. 1011 



3. Pseudociihus hexajrylus, n. sp. (PI. 94, fig. 12). 



Mitral ring circular, two-thirds as broad as the subcircular, or nearly square, basal ring ; both 

 rings with a simple spine on the two sagittal corners (dorsal and ventral), with a forked or branched 

 larger spine on the two lateral corners (right and left). All twelve rods more or less curved and thorny. 



Dimensions. — Breadth of the mitral ring 0'07, of the basal ring O'l. 



Ealitat. — Central Pacific, Station 272, depth 2600 fathoms. 



Genus 438. Lithocubus,^ Haeckel, 1881, Prodromus, p. 447. 



Definition. — T y m p a n i d a with two simple horizontal rings of equal size, 

 connected by four parallel vertical columellaj. 



The genus LitJiocuhus is very remarkable for the regular cubical form of the 

 skeleton, which is composed of twelve siliceous rods, corresponding to the twelve edges 

 of a cube. Eight of these are horizontal, and enclose the two parallel square horizontal 

 gates, the superior mitral and the inferior basal gate. The four other rods are vertical, 

 parallel, and connect (as lateral edges of the cube) the corresponding corners of the two 

 horizontal squares. Two opposite ones of these four vertical columellse are the remaining 

 halves of the incomplete sagittal ring ; the two alternate are the remaining halves of the 

 incomplete frontal ring. The four lateral gates between these four columellse are also 

 square or rectangular, and either of the same size as the two horizontal gates, or some- 

 what larger. Lithocubus may be derived from Acrocubus by reduction of the coryphal 

 and the basal part of the primary sagittal ring, the only remaining parts of which are 

 the anterior and the posterior columellse. 



1. Lithocubus geometricus, n. sp. (PI. 94, fig. 13). 



The twelve rods, corresponding to the edges of the geometrical cube, are straight and smooth ; 

 the eight corners provided with a smaU conical thorn. The shell exhibits six equal square sides, 

 and represents therefore exactly the geometrical form of a cube. 



Dimensions. — Diameter of the cube 0'05 ; thickness of the bars 0"008. 



Hahitat. — -Tropical Pacific, Station 224, depth 1850 fathoms. 



2. Lithocubus octacanthus, n. sp. 



The twelve rods of the cubical shell are shghtly curved, convex, smooth, as in the similar 

 preceding species. It differs from that in the development of eight slender radial spines, arising 

 from the eight corners of the geometrical cube, from two to three times as long as its diameter, 

 and lying opposite in pairs in its diagonals. 



Dimensions. — Diameter of the cube 0"06 ; length of the spines 0"15. 



Habitat. — North Atlantic, Canary Islands, surface. 



^ Lithocubus=C\i\ie oi silex; 'kih;, xifios. 



