84 



THE VOYAGE OF H.M.S. CHALLENGER. 



Chorological Synopsis of the Species. 



1. Plutonaster hifrons, "Wyville Thomson, sp. (PL XL figs. 1-4 ; PL XIII. figs. 9 

 and 10). 

 Archaster lifrons, Wyville Thomson, 1873, The Depths of the Sea, p. 122, figs. 17 and 74. 



Rays five. R = 90 mm. ; r = 21 mm. R< 4'5?\ Minor radial proportion 233 per 

 cent. Breadth of a ray near the base, 25 mm. 



Rays elongate, moderately broad at the base, tapering continuously to a finely pointed 

 extremity. Interbrachial arcs widely rounded. Lateral walls vertical. Abactinal surface 

 plane, or slightly inflated. Actinal surface subplane, highest at the mouth-angles, and 

 sloping thence to the margin and along the rays. 



The paxillae of the abactinal surface are minute and crowded ; and are composed of 

 twenty to twenty-five short papilliform spinelets — five or six in the midst of each crown 

 beino- more robust than the rest. 



The marginal plates are large, well-defined, and conspicuous. The supero-marginal 

 plates, thirty-three in number from the median interradial line to the extremity, form a 

 well-defined border on the abactinal surface. They are about as broad as high, and covered 

 with granules, which become subcorneal in form on the lateral half of the plate. Each 

 supero-marginal plate bears a single, moderately long, conical pointed spinelet, which 

 stands on the rounded angle of the plate, and is directed outward almost horizontally. 

 Sometimes this spinelet may be redoubled on two or three plates near the middle of the 

 ray. The sutures between the plates are well-defined. 



The infero-marginal plates correspond to the superior series, and are similar in every 

 respect, covered with similar subconical granules, and each with a similar and equal-sized 

 spinelet directed horizontally. The supero-marginal spines diminish in size towards the 

 interbrachial arc and towards the extremity, and also the infero-marginal spines, but in a 

 less degree. The largest spines are consequently about midway on the ray. 



1 The exact station is not known : an alternative record being given. 



