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



evolution has been a gradual sinking of the plates supporting 

 the ventral sac into the dorsal cup. It is therefore perfectly 

 logical to conclude that the original position of x was, where 

 we see it in Iocrinus and Merocrin us, resting on the left upper 

 slope of either the right posterior radial or its costal. To 

 these earlier forms then we must turn to discover its homo- 

 logies, and we immediately see that in size and position it is 

 just like the adjacent arm-plate. Further the series of ossicles 

 that follows it in many forms can hardly be distinguished 

 from a brachial series; e. g„ Iocrmus f Merocrinus } Heterocrinus. 

 This view is confirmed by the evolution of the Calceocrinidai 

 as gathered from a recent paper by Dr. Eugene N. S. Ringue- 

 berg* ; in this family the earliest genus Castocrinus (PI. XIV. 

 fig. 9) shows a series of anal plates supported by the first costal 

 of the left posterior radius, which is exactly comparable to the 

 arm-bearing costal of the right posterior radius ; Proclivo- 

 crinus (PL XIV. fig. 10) shows the intermediate stage between 

 this and Calceocrinus (PI. XIV. fig. 19). There can be little 

 doubt that there is here presented the whole process of the 

 transformation of an arm into a support for the ventral sac ; 

 the ancestor of Castocrinus must have had simply 5 arms. It 

 will be noticed that in Castocrinus the anal plates arise from 

 the left posterior arm and not from the right (I have assumed 

 that Dr. Ringueberg's figures are not reversed, in the text he 

 makes no mention of "right" or "left"), consequently I 

 merely adduce the case to show the possibility of the process 

 and the tendency of the Fistulata to accomplish it, if not by 

 one means, then by another. To the exact homologies of the 

 plates in the Calceocrinidaj I shall return when dealing with 

 that family ; I agree with Dr. Ringueberg in his rejection of 

 Wachsnmth and Springer's nomenclature, but differ from 

 him on a few points (see PI. XIV. figs. 9,. 10, 19). My con- 

 clusion therefore is that x originated as a plate morpholo- 

 gically corresponding to an ordinary brachial ; and, for this 

 reason, I propose to distinguish it as the " Brachianal." 



The Brachianal and the Radianal are not only morpholo- 

 gically distinct, but they were evolved in fulfilment of distinct 

 physiological aims. Their differing functions have already 

 been alluded to ; the brachianal primitively serves to support 

 the ventral sac, while the radianal serves to enlarge the space 

 between the right and left posterior radii. Of course the 

 radianal does come to share in the support of the sac, when 

 t sinks down on to its summit ; while the brachianal, as it 

 sinks and widens, does its part in enlarging the anal area. 



* "The Calceocrinidae &c," Ann. N. York Acad. Sci. vol. iv., Article 

 xvii. plates x. and xi. 1889. 



