224 ECHINODERMS OF THE BRITISH ISLES 



Another, Ophionereis Liitken, is represented in European seas 

 by the species 0. reticulata Liitken, which has been found at the 

 Azores (also at the Cape Verde Islands), at depths of 225-500 m. 

 There is scarcely any prospects of this mainly littoral West 

 Indian species occurring as far north as the British coasts. (The 

 genus Opiiionereis is very easily recognised by having a supple- 

 mentary plate on each side of the dorsal arm plates.) 



1. Ophiochiton Lyman. 



Dorsal side of disk usually with a series of larger marginal 

 scales. Dorsal arm plates without supplementary plates. Arms 

 usually with a keel along ventral and dorsal mid-line. A well- 

 developed infradental papilla, the outermost mouth papillae 

 enlarged. 



Only one species known from the British seas, but another, 

 Ophiochiton solutus Koehler, has been found off the Azores and 

 Madeira, 1139-2220 m. ("Talisman"), and may thus well be 

 expected to occur also in the deep sea off the British coasts. 



Key to the species of Ophiochiton Jcnown from or likely 

 to occur in the British seas. 



Mouth shield broader than long, triangular, with a small outer lobe ; 

 no keel along dorsal side of arms . . O. solutus Koehler ^ 



Mouth shield not longer than broad, somewhat spearhead-shaped, 

 with a more or less conspicuous outer lobe ; an indistinct keel 

 along dorsal side of arms . . 1. O. ternispinus Lyman 



1. Ophiochiton ternispinus Lyman. (Fig. 127.) 

 (Sj'U. Ophiochiton tenuispinus Lyman.) 



Primary plates and several other, somewhat minor, plates 

 very conspicuous in the general small scaling on the dorsal side 

 of disk. Radial shields small, wholly separated. Dorsal arm 

 plates very broad, with slightly convex outer edge, broadly con- 

 tiguous, somewhat elevated in the middle towards the outer edge, 

 which produces a slightly keeled aspect ; ventral plates likewise 

 broadly contiguous, with somewhat produced outer corners and 

 convex outer ed^e. A rather distinct keel along ventral mid-line 

 of arm. Three arm spines, the upper one the longest. Two 

 tentacle scales, the outer one the larger, and more or less com- 



^ The difference between O. solutus and ternispinus is so inconsiderable 

 that it would seem rather probable that O. solutus is really identical with 

 ternispinus. 



