96 ECHIN0DERM8 OF THE BRITISH ISLES 



G. A single or double series of larger marginal paxillae. Oral inter- 

 radial j:)lates ]:)rosent. Spines on adambulacral plates in two 

 series at right angles ... V. Fam. Solasteridce 



No larger marginal paxilhip. Oral interradial plates absent or 

 indistinct ......... 7 



7. Arms 5,^ short ; spines on adambulacral plates forming together 

 with those of lower marginals a single transverse series, not 

 webbed .... IV. Fam. Korethrasteridce 



Arms 6 or more, long, slender ; adambulacral spines webbed, not 

 associated with those of lower marginals Fam. Myxasteridce 



I. Family Radiasterid^ 

 (Syn. Mimasteridce.) 



Dorsal side covered with paxillse, borne by small, mostly 

 independent plates. Upper and lower marginals small, but 

 distinct, paxilliform, forming a distinct edge to the body, de- 

 limiting the dorsal from the ventral side. Oral interradial 

 plates in transverse series, imbricated, with tufts of spinelets 

 (paxillae). 



This family is sometimes placed among the Phanerozonia, 

 to which it has no doubt a close relation ; the author, however, 

 thinks it more naturally j^laced among the Spinulosa. In any case, 

 it has near relations to both orders, and should perhaps rather be 

 regarded as a connecting link between these two orders. 



Only one genus represented in British (and European) seas. 



1. Radiaster Perrier. 

 (S^Ti. Mimaster Sladen.) 



Plates of the dorsal side, also those at the sides, simple, not 

 imbricating. Tube-feet with well-developed sucking disk. Geni- 

 tal organs confined to the disk. Hepatic caeca with long sub- 

 divisions, so that each ray appears to have 6-10 separate caeca 

 of unequal length. 



(The only other genus known of this family, Mimastrella 

 Fisher, with the species M. {Mimaster) cognatus (Sladen) from 

 the S.W. Atlantic, differs from Radiaster in the lateral dorsal 

 plates being four-lobed, imbricated, in the tube-feet having only 

 a rudimentary sucking disk, the genital organs extending for 

 over half the length of rays, and in the caeca being simple. There 

 is not much probability that this form will occur in the N.E. 

 Atlantic.) 



Only one species known from the British (and European) seas. 

 ^ Exceptionally there may be 6 arms in Korethraster . 



