76 ECHIN0DERM8 OF THE BRITISH ISLES 



Three families represented in the British seas. Another 

 family, Ophidiasteridce, is represented in the Mediterranean, 

 along the Moroccan coast and at the Azores, by the two genera 

 Ophidiaster and Hacelia, each with one species, 0. ophidianus 

 Lamarck and H. attenuata Gray. Both are, however, shallow- 

 water forms, and there is hardly any probability of their occurring 

 in the British seas. They are therefore not included in the 

 key. 



The suborder Valvata is perhaps not a very natural one. The 

 Poraniidae form a transition to the Spinulosa, and also the 

 Ophidiasteridae are rather distant from the Goniasteridae. 



Key to the families of the suborder Valvata known from 

 the British seas. 



1. A large recurved hj^aline spine (or a pair of such spines) usually 



occurs on the jaws (lacking in the genus Hoplaster). Arms 

 short, body rather pentagonal. Marginal plates well developed, 

 covered with spines ; an odd interradial marginal plate 

 present . . . . .1. Fam. Gnathasteridce 



No recurved hyaline spine on jaws ; marginal plates naked or 

 with grains or spines, but these latter are never long, slender, 

 and movable ; no odd interradial marginal plate . . 2 



2. Marginal plates large and distinct, not covered by a thick skin ; 



edge of disk vertical, not sharp. Dorsal side covered with 

 flat spines (or grains), arranged in the form of paxillae, or with 

 scattered large spines surrounded by grains 



II. Fam. GoniasteridcB 



Marginal plates smaller, more or less indistinct, carrying spines 



or covered with naked skin. Edge of disk sharp. Dorsal 



side covered with a thick, naked skin, or with fine, or even 



microscopical, spines . . . III. Fain. Poraniidce 



I. Family Gnathasterid.e 

 (Syn. Odontasteridce.) 



Marginal plates usually large and distinct ; an odd plate in 

 the middle of the interradii. Dorsal side flattened, with mostly 

 short spines or granules, arranged more or less distinctly like 

 paxillae. Usually one or two (rarely three) large, recurved spines, 

 ending in a hyaline point, on the jaws. Papulae mainly in the 

 radii. Simple pedicellariae may be present. Genital organs (so 

 far as known) only at base of arms. 



This family is especiall}' richly represented in Antarctic seas, 

 while in the Atlantic only two genera are known to occur, one of 

 these, Odontaster Verrill, being represented in the British seas ; 



