284 ECHINODERMS OF THE BRITISH ISLES 



west coast of Trcland, from off Arraii Islands, and off Rockall.^ 

 In places it occurs in very great luinibers, and, upon the whole, 

 appears to be rather common, occurring in depths of ca. 500- 

 900 m. It is elsewhere distributed from the Denmark strait to 

 off the Portugal coast, and on the American side at least down to 

 the Florida coast. Bathymetrical distribution ca. 145-900 m. 



3. HygTOSoma Mrtsn. 

 (Syn. Echinosoma Pomel.^) 



Test very flexible ; no sharp limit between oral and aboral 

 side, the aspect of the two sides not very different. Ambulacra 

 consisting of a large j)riniary plate which is simple, not divided 

 into an inner and an outer plate, and of two much smaller 

 plates lying close to the edge of the area (Fig. 156, 2). Tube- 

 feet on the oral side in a single series, on the aboral side in three 

 close series. Primary spines of oral side ending in a large, white 

 hoof. Tridentate pedicellarise with involute or simply leaf- 

 shaped valves. No tetradactylous pedicellariae ; ophicephalous 

 pedicellarise may occur. 



Two species have been recorded from the British seas (and 

 the N.E. Atlantic). 



Key to the species of Hygrosonia known from the British 

 {and European) seas. 

 Valves of tridentate pedicellariae strongly involute, more or less 

 curved . . . . . I. H. Petersii (A. Agass.) 



Valves of tridentate pedicellariae simply leaf -shaped 



H. uranus (Wyv. Tli.) 



1. Hygrosoma Petersii (A. Agassiz). (Fig. 161.) 



(Syn. Phormosoma Petersii A. Agass. ; Ph. luculentum Koehler, 

 W. de Morgan, non A. Agassiz ; Phormosoma uranus Koehler, 

 Bell, non Wyv. Thomson ; Echinosoma Petersii (A. Agass.).) 



Large tubercles of the oral side only on the outer 5-6 plates, 

 arranged in rather irregular series, two in the ambulacral, four 

 in the interambulacral areas. On the aboral side the tubercles 

 quite irregularly arranged. Ko membranous spaces between the 



1 The locality " North Sea " given for this species in H. L. Clark's 

 Catalogue oj the Recent Sea-urchins of the British Museum must rest on an 

 error in the labelling. No Echinothurid occurs in the North Sea. 



2 The name Echinosoma cannot be used for this genus, being pre- 

 occupied several times over. 



