Mr. F. A. Bather on British Fossil Crinoids. 331 



But in the beginning, physiologically no less than morpho- 

 logically, the two plates were distinct. 



It will be observed that the position now taken with regard 

 to the brachianal is not precisely that held by Messrs. Wachs- 

 muth and Springer in 1879. They then inclined to regard 

 the whole ventral sac as a metamorphosed arm, and, oblivious 

 of the fact that locrinus possessed no pinnules, they compared 

 the structure in that genus to a series of brachials supporting 

 coalesced pinnules. This view is as unnecessary as it is un- 

 tenable. The brachianal itself and the plates that follow it 

 in regular series, which may be called 2nd, 3rd, &c. brachi- 

 anals, are indeed to be regarded as brachials ; but the remain- 

 ing plates of the sac are nothing more than plates deposited 

 in an extension of the ventral perisome. Such an extension 

 of the ventral perisome up an arm is no uncommon thing in 

 Palaeozoic Crinoids; Onychocrinus is especially characterized 

 by it, while in the Fistulata it is well shown by the common 

 Botryocrinus of Dudley. Even more to the point is Angelin's 

 description of Cyathocrinus (= Gnorimocrinus) interbrachia- 

 tus * : — " Basis tubi ventralis cum brachio connata," and the 

 accompanying illustration. 



"What light the foregoing explanation of the anals of the 

 Fistulata may throw upon the anal plates of Crinoidea in 

 other suborders cannot here be considered. There is no doubt 

 something to be said for applying it to the Ichthyocrinida3, 

 while in Thaumatocrinus the peculiar anal appendage may 

 represent a series of brachianals that have lost their con- 

 nexion with the anal tube. But in these forms the presence 

 of interradials complicates the problem. Often too has the 

 anal series of Reteocrinus and allied genera been compared to 

 a 6th arm ; but even accepting that view there would be no 

 actual homology between brachianals derived from the right 

 posterior radius, and brachianals derived from a posterior 

 radius unrepresented in the Fistulata. Until then I have 

 carefully examined all the variations of the anal area pre- 

 sented by other Crinoids I can express no opinion on this 

 matter. My present business is with the classification of 

 the Fistulata. 



* ' Iccmographia Crinoideorum,' p. 23, pi. xxix. fig. 78. 



24* 



