282 ECHINODERMS OF THE BRITISH ISLES 



1. Arceosoma hystrix (Wyv. Thomson). (Fig. 159.) 



(Syn. Calveria hystrix Wyv. Thomson ; Asthenosoma hystrix 

 (Wyv. Thomson).) 



A regular series of large tubercles along the outer (adradial) 

 edges of the interambulacra, and another, somewhat less regular, 

 series along the middle of the interambulacral plates. Irregular 

 series of similar large tubercles in the ambulacra. The plates of 



Fig. 159. — Arceosoma hystrix ; oral side. Slightly reduced. 



both areas generally not restricted in the middle, leaving no 

 distinct membranous spaces between them. The large tridendate 

 pedicellarise (Fig. 158, 2) with the valves much involute, the 

 small widened terminal part with straight, more or less irregularly 

 serrate edges ; they reach a size of ca. 1-2 mm. length of head. 

 Also smaller tridentate pedicellariae occur, with the involuted part 

 of the valve shorter and the widened terminal part correspond- 

 ingly larger. No tetradactylous pedicellariae. Colour in life a 

 bright red or claret colour, which keeps very well in alcohol and 

 also in dried specimens ; even the denuded test retains the red 

 colour. Reaches a size of ca. 240 mm. in horizontal diameter. 



