1890.] NATURAL SCIENCES OF PHILADELPHIA. 377 



faces of adjoining basals, its upper face supporting the first plate of 

 the tube, which in some of these genera is partly enclosed in the 

 calyx. To understand the relations of this plate, it is very impor- 

 tant to note that throughout the various modifications which the 

 posterior side of the Fistulata undergoes in geological times, this 

 plate retains invariably its alternate arrangement with the basals. 

 The plate, therefore, was radially disposed from the beginning, and 

 always remained so. This shows that the widening of the anal area, 

 wliich took place in later fonns, owing to the increasing width of 

 the ventral sac, was effected by a displacement of the arm-bearing 

 plates. The oblique position, which the radially situated anal 

 plate holds toward the posterior basal and the other anal plate, and 

 this toward the first plate of the tube, gives to these plates a sort of 

 alternate arrangement, which is continuous throughout the whole 

 tube. 



The relations of the anal plates in the various groups of the Fis- 

 tulata were discussed by us at different times, and a comparison of 

 the literature will show that our views underwent some modification 

 in the course of time. ^ 



The subject was also discussed by Dr. P. H, Cai'penter in his 

 paper " On the Relations of Hybocrinus, Baerocrinus and Hyho- 

 cystites"'^ 



We tliink it unnecessary to give a review of all these ])apers, and 

 we can dispense with it more readily, as Mr. F. A. Bather, in an in- 

 teresting paper " On the British Fossil Crinoids,"^ has lately given 

 a full history of them. We, therefore, take up the question where 

 Ave left it in 1885. 



Mr. Bather, in the paper above cited, has advanced views re- 

 specting the origin of one of the anal plates, from which we regret 

 to be obliged to dissent. He agrees with us and Carpenter that the 

 radial anal plate, the so-called azygous piece, constitutes primarily 

 the lower portion of the right posterior radial, which in the earlier 

 forms occupies a position immediately below the radial ; but he dif- 

 fers from us essentially upon the origin of the other anal plate. He 

 seeks to prove that this plate " originated as a plate morphologi- 



1 1879, Revision, Pt. I, pp. 71 and 72; 1883, Amer. Jour. Sci., Vol XXVI, 

 pp. 365 to 377; 1885, Revision, Pt. Ill, Sect. I, pp. 11, 12 and 40; and 1886, 

 ibid., Sect. II, pp. 196 and 210. 



2 Quart. Journ. Geol. See. London, Vol. XXXVIII, pp. 398-312. 



3 Ann. and Mag. Nat. Hist. (6ih ser.). Vol. V, April, 1890, pp. 319-334. 



