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



to have disappeared completely, so that there remain only twenty aspinal pores 

 (PI. 139, fig. 4). 



The internal cavity of the shell in all Hexalaspida is very small, on account of the 

 thickness of the massive wall ; the latter is often greater than the diameter of the 

 cavity. Setting aside this disproportion, the space of the cavity is further much reduced 

 by the internal parts of the thick radial spines, which are united in the centre by their 

 pyramidal bases (PL 140, fig. 15). Usually the twenty bases seem to remain free 

 (supported one upon another by means of their triangular faces); but sometimes they 

 seem to grow together perfectly and to form a single central star of acanthin. In some 

 species, too, the sutures of the meeting branches of the apophyses of neighbouring 

 spines seem to grow together, so that the whole shell exhibits a single piece of acanthin. 



The Central Capsule of the Hexalaspida is therefore very small and seems to fill 

 up the greatest part of the shell-cavity. Its form is constantly more or less 

 lenticular, sometimes lentelliptical. On account of the opacity of the shell I could not 

 make out its shape more closely. 



Synopsis of the Genera of Hexalaspida. 



All twenty spines externally developed, \ Sheaths'of the spines not prominent, . . 376. Hexalaxjti--: 



prominent on the surface of the / 

 lenticular shell. j Sheaths of the spines prominent, . . 377. Hfjcaeomis. 



Only six spines (the hydrotomical 

 spines) externally developed and 



Sheaths of the spines not prominent, . . 378. 



prominent on the margin of the 



disk; the other fourteen spines not j Sheaths of the spines prominent, . . 379. Hexacolpnn. 



prominent. 



Genus 376. Hexalaspis, 1 n. gen. 



Definition. H exalaspida with twenty prominent radial spines, which are not 

 surrounded by prominent sheaths ; the six hydrotomical spines much larger than the 

 fourteen others. 



The genus .Hexalaspis is the simplest form among the Hexalaspida, and may be 

 derived directly from Dictyaspis among the Belonaspida, by stronger development of 

 the six hydrotomical spines. As in the following genus Hexaconus all twenty spines 

 are prominent externally. 



Subgenus 1. Hexalasparium, Haeckel. 

 Definition. Six hydrotomical spines of equal size. 



1 Hexalavpis Shield with six wings ; 



