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



twenty spines are provided with prominent basal sheaths, which are truncate conical, sulcate, and 

 dentate on the narrowed distal mouth. The sheaths of the six hydrotomical spines are twice 

 to three times as large as those of the fourteen smaller spines. 



Dimensions. — Diameter of the shell 0'15 ; length of the spines 0'12; basal breadth of the 

 equatorial spines 0'04, of the other spines 0"02 ; length of the hydrotomical sheaths 0'05, of the 

 other sheaths 0-02. 



Habitat. — Central Pacific, Station 274, surface. 



6. Ilexaconus echinatus, n. sp. (PI. 140, fig. 12). 



Six hydrotomical spines of unequal size; the two equatorial, and their sheaths twice as large as 

 the four polar spines. These six spines are six-edged, pyramidal, and their basal half enveloped 

 by very large conical sheaths which are sulcate, and twice as broad on the dentate distal mouth 

 as on the narrower base. The other fourteen spines are very thin, two-edged, half as long, with 

 low sheaths. Approaches some forms of Diploconus. 



Dimensions. — Diameter of the shell 015 ; length of the equatorial spines 0'13, of the piolar 

 spines 0'08. 



Habitat. — Indian Ocean, BelHgemma, Ceylon, Haeckel, surface. 



Genus 378. Hexonaspis^ n. gen. 



Dejinitlon. — H e x a 1 a s p i d a with six 23romiueut radial spines (in the hydrotomical 

 plane) which are not surrounded Ity prominent sheaths ; the fourteen other spines c^uite 

 rudimentary, not jjrominent. 



The genus Hexonaspis and the following Hexacolpus differ from the two preceding 

 genera in the rudimentary shape of the fourteen reduced and stunted smaller spines ; 

 these are only developed inside the shell, and are not prominent outside over its surface. 



Subgenus 1. Hexonasparium, HaeckeL 

 Definition. — Six hydrotomical spines of equal size. 



1. Hexonaspis heliosestrum, n. sp. 



All six hydrotomical spines of nearly equal size, isosceles triangular, compressed, smooth, 

 about as long as the diameter of the shell and three times as long as broad at the base. Tliis 

 species is very similar to Hcxalaspis heliodiscus (PL 139, fig. 2), but differs in the lai'ger size of the 

 six marginal spines and in the complete external absence of the fourteen smaller spines. 



' Hexonaspis = Shield with six auricles ; tjojiof, aerx/'j. 



