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



7. Hexapyle polyacantha, n. sp. 



Cortical shell circular, thorny, with a circle of numerous conical radial spines in the periphery ; 

 its diameter nearly three times as great as that of the medullary shell. Three half -girdles half . as 

 broad as the three kidney-shaped gates on each side of the discoidal shell. 



Dimensions. Diameter of the cortical shell O'll, of the medullary shell 0'04; breadth of the 

 gates 0'04, of the bridges between them 0'02. 



Habitat. Pacific, central area, Station 271, depth 2425 fathoms. 



Genus 250. Pylodiscus, 1 n. gen. 



Definition. P y 1 o d i s c i d a with Triopyle-shaped medullary shell, surrounded by 

 three distal arm-chambers. Notches between the three arms closed by lattice-work and 

 by an equatorial girdle. 



The genus Pylodiscus differs from Hexapyle in the development of two convex plates 

 of lattice-work, which close the six open gates and transform the disk into a biconvex 

 fenestrated lens. Pylodiscus simulates therefore the characteristic form of Triodiscus; 

 but whilst in the latter we find only three arm-joints and three simple gates, their number 

 is doubled in the former. 



1. Pylodiscus triangularis, n. sp. (PI. 48, fig. 17). 



Cortical shell triangular, with rounded corners, three times as broad as the triangular, Triopyle- 

 shaped medullary shell. Three arms about as broad as the three egg-shaped gates between them. 

 (Differs from Hexapyle triangula only by the delicate network of both triangular convex covering 

 plates, which close the open gates.) Six larger and many smaller spines on the margin of the disk. 



Dimensions. Diameter of the disk 0'15, of the gates 0'05. 



Habitat. Pacific, central area, Station 270, depth 2925 fathoms. 



2. Pylodiscus sexangularis, n. sp. 



Cortical shell hexangular, equilateral, four times as broad as the circular, Triopyle-shoped 

 medullary shell. Surface spiny. Three arms nearly of the same form and size as the three 

 triangular gates between them. (Similar to Hexapyle sexangularis, but differing in the irregular 

 loose network closing the gates.) 



Dimensions. Diameter of the disk 0*18, of the gates 0'06. 



Habitat. North Pacific, south of Japan, Station 237, surface. 



1 Pylodiscus Disk with gates; vvhy, SiVxo;. 



