156 ECHINODERMS OF THE BRITISH ISLES 



articulation, and the Zygophiurce, in which the articulating 

 surface bears various processes and pits. It has the great fault 

 that the difference between the Streptophiurae and Zygophiuras 

 is not real — in fact, the vertebra? of the " Streptophiurae " are of 

 the same type as those of the " Zygophiurae," whereas the 

 difference in regard to the vertebral articulation between the 

 Zygophiurae and the Cladophiurae appears to be real enough and 

 to mark two main groups of the brittle -stars. Another, more 

 recent classification, proposed by Matsumoto, is mainly founded 

 on differences in the shape of the first vertebrae and the articu- 

 lation of the genital plate with the radial shield ; it divides the 

 Ophiurans into the following orders : Phrynophiurce, Lcemo- 

 phiurce, Gnathophiurce, and Chilophiurce. 



The present author cannot accept this classification ; especi- 

 ally^ the Phrynophiuridae seem a very artificial group, containing 

 both the Ophiomyxidae and the Euryalids. The Ophiomyxidae 

 may well be regarded as the most primitive of Zygophiurids, but 

 they are probably not nearly related to the EuryaUds, with quite 

 a different type of vertebrae. The more natural classification, 

 therefore, would be the old one, dividing the Ophiuroids intp the 

 two orders: OpJiiurce (= Zygophiurae) and Euryalce (= Clado- 

 phiurae). Which of these groups is the more primitive is hardto 

 say in our present state of knowledge. If the Euryalids are 

 placed first here, this does not mean to say definitely that 

 the author would regard them as the less specialised type. 



Key to the Orders of Ophiuroids. 



The arms are moved vertically ; articulation of vertebrae by means 

 of hourglass -shaped surfaces. No distinct scale -covering, but 

 disk and arms covered by a thick soft skin or a mosaic of 

 granules. Spines downwardly directed. Arms simple or 

 branched . I. Euryalce 



The arms are as a rule moved horizontally ; articulation of vertebrae 

 by means of various pits and processes. Mostly a distinct 

 scale-covering on disk and arms, more rarelj^ a soft skin covers 

 the scales. Arm spines not downwardly directed ; arms simple 



II. Ophiurce 



I. Order. Euryal^ Miiller and Troschel. 



(Syn. Astrophiurce, Cladophiiirce Bell.) 



Disk and arms covered by naked skin or by a close coat of 

 granules, more rarely with distinct scales. Ventral and dorsal 

 plates of arms lacking or more or less rudimentary, covered by 



