ECHINOIDEA 277 



One family, the Echinothuridce, is represented in the British 

 seas, but three more are, more or less, likely to occur there also, 

 viz. the Diadematidce, Aspidodiadematidce, and Pedinidce. Of the 

 family Diadematidse the genus Centrostejjhanus Peters is repre- 

 sented by the species C. longisj^i^ius (Philippi) at the Azores and 

 Canaries and in the Mediterranean ; it is mainly a sublittoral 

 form, occurring in depths of ca. 50-200 m. There is perhaps 

 a possibiHty of its occurring in the deeper sublittoral off S.W. 

 Ireland. The genus Diadema Gray, represented at Madeira and 

 the Canaries by the species D. antillarum Philippi, cannot be 

 expected to occur as far north as the British area, being a strictly 

 littoral form. Of the family Aspidodiadematidse, the genus 

 Plesiodiadema Pomel (Syn. Dermatodiadema A. iVgassiz), namely 

 the species PL antillarum (A. Agassiz), has been found south of the 

 Canaries, 2500 m. (" Valdivia ") ; some unidentifiable fragments 

 of a Plesiodiadema were taken by " Hirondelle II." at 30° 50' N., 

 25° 43' W., 5300 m. The species PI. antillarum is elsewhere known 

 from the West Indies, 280-2860 m. It is perhaps not unlikely that 

 the other West Indian Aspidodiadematid, Aspidodiadema Jacobyi 

 A. Agassiz (ca. 170-515 m.), also will prove to occur in the N.E. 

 Atlantic. 



The family Pedinidse is represented in the N.E. Atlantic by the 

 genus Ccenopedina A. Agassiz, the species C. cubensis A. Agassiz 

 (Sjni. Hemijjedina cubensis (A. Agassiz)) having been found 

 off the Azores and Canaries, 1098-1187 m. ('' Princesse Alice ") ; 

 the species is elsewhere known from theWest Indies, ca. 250-485 m. 



The main characters of these three families are given in the 

 following key. For a description of the species in question 

 reference may be made to Koehler's Faune de France : Echino- 

 dermes, p. 112 {Centrostephanus longispinus) ; Monaco. Fasc, 

 xxxiv., p. 221 ; Pis. I., XXX., XXXI. {Ccenopedina cubensis) ; 

 and A. Agassiz : '" Blake " Echini, p. 25, 27, Pis. IX., IX.a {Plesio- 

 diadema antillarum, Aspidodiadema Jacobyi). The genera Plesio- 

 diadema and Aspidodiadema are distinguished by the latter having 

 some large primary tubercles in the ambulacra, while in the former 

 all the ambulacral tubercles are small. 



Key to the families of the Aulodonta hnown from or likely to he 



represented in the British seas. 



1. Only one pair of ambulacral plates on the peristome in each 



radius, the ji^ristome otherwise naked or with irregularly 



arranged small plates ; test not soft or flexible . . 2 



A series of ambulacral plates on the peristome in each radius, the 



peristome being completely covered by the ambulacral plates ; 



test more or less soft, flexible . . I. Echinothuridce 



