ECHINOIDEA 



283 



This splendid species, one of the great discoveries of the 

 " Porcupine", appears to be common in the warm area of the 

 Faroe Channel and off the west coast of Ireland ; it is a charac- 

 teristic inhabitant especially of the ooze between ca. 900-1200 m. 

 It is elsewhere distributed from off S.W. Iceland to the Bay 

 of Biscay, the Azores, and the Canary Islands. Bathymetrical 

 distribution ca. 360-1525 m. 



2. Arceosoma fenestratum (Wyv. Thomson). (Fig. 160.) 



(Syn. Calveria fenestrata Wyv. Thoms. ; Asthenosoma fenes- 

 tratum (Wyv. Thoms.), Arceosoma violacenm Mrtsn.) 



In general arrangement of tubercles 

 and spines resembling the preceding 

 species very much ; but usually the 

 plates are restricted in the middle, 

 leaving a more or less conspicuous 

 membranous sj)ace between them. 

 The large tridentate pedicellarise (Fig. 

 158, 3) have the edge of the outer, 

 widened part of the valves deeply 

 sinuate ; they are very conspicu- 

 ous, up to 2 mm. length of head. 

 Smaller forms are found in consider- 

 able numbers, with the involuted 

 part shorter and the widened part 

 correspondingly larger, till, in the 

 smallest ones, the valves become simply 

 leaf -shaped. The tetradactylous pedi- 

 cellarise (Fig. 160) are of relatively rare 

 occurrence and rarely found intact. 

 Colour brownish or purplish-violet or 

 pale mauve, keeping fairly well in 

 alcohol and also in dried specimens. 

 Reaches a larger size than A. hystrix, 

 up to ca. 280 mm. horizontal diameter 

 of test. 



The shape of the large tridentate 

 pedicellarise and the general colour are 

 the most reliable characters distinguishing this species from 

 A. hystrix. 



In British seas this species has been recorded from off the 



Fig. IGO. — Tetradactylous 

 pedicellaria of Arceosoma 

 feneatratum. x 24. (After 

 Wyv. Thomson, " Por- 

 cupine " Echinoidea.) 



