126 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



further research. The subject was also discussed by Dr. P. H. Carpenter in 

 his paper '' On the ReLitions of Hylocrinus, Baerocrinus and Hyhocystites,'^ 

 and lately by Mr. F. A. Bather in a paper " On the British Fossil Crinoids."t 

 Bather agrees with us and with Carpenter that the radially disposed plate 

 is a posterior inferradial, which in some groups took on anal functions. 

 He gave it the name " radianal/' which we have accepted, but w^e 

 apply the term only when the plate actually serves as a support of the 

 tube. Eespecting the origin of the other plate, which we take to be a 

 special interradial, he advances views from which we regret to be obliged 

 to dissent. His idea is that this plate " originated as a plate morphologically 

 corresponding to an ordinary brachial/' and he undertakes to prove that in 

 its pal^eontological development it passed down from above the radials to the 

 basals, and between the radials. He calls it a '' brachianal," — a term which 

 becomes meaningless if it proves to be interradially disposed. To this plate 

 w^e apply the general term '' anal plate," as we take it to be the homologue 

 of the anal plate of the Antedon larva, and structurally identical with the 

 first anal plate of Actinocrinus. 



We give, on Table C, a series of diagrams to illustrate the development 

 of the anal plates, in which the ^'brachianal" — of Bather — is marked x. 

 The radials are designated by the letter R, and when compound, the lower 

 section — the inferradial, which is also the '' radianal " w^hen it helps to 

 support the tube — by E. The tube plates are called t. 



To the Plate R w^e have already alluded in the chapter on the radials, 

 discussing those forms in which it represents a part of the radial, and lies 

 in a vertical line with R. Let us now consider those forms in which it serves 

 as anal plate. Among these forms, of which Poteriocrinus may be regarded 

 as the type, four of the ^ive radials are simple, and the Plate x, which is gen- 

 erally represented, rests upon the basals. The ventral sac, which in the ear- 

 lier forms was rather insignificant, had rapidly increased in size at the close 

 of the lower Silurian, in such a manner that the sloping upper faces of the 

 radials were insufficient to support it, and certain changes in the structure of 

 the dorsal cup became inevitable. It thus happened that the two posterior 

 radials, which had previously been in contact laterally (locrinus, Fig. 9) were 

 now parted, and the Plate x was introduced to fill the vacant space {Ben- 

 drocrinus, Fig. 13). These modifications, however, did not affect the position 



* Quart. Journ. Geol. Soc. London, Vol. XXXVIII., pp. 298-312. 



t Ann. and Mag. Nat. Hist. (6th Ser.), Vol. Y., April, 1890, pp. 319 to 334. 



