126 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
further research. The subject was also discussed by Dr. P. H. Carpenter in 
his paper “On the Relations of Hybocrinus, Baerocrinus and Hybocystites,* 
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 we 
apply the term only when the plate actually serves as a support of the 
tube. Respecting 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 paleontological 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 
we 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 2. 
The radials are designated by the letter A, and when compound, the lower 
section — the inferradial, which is also the “ radianal”’ when it helps to 
support the tube —by f’. The tube plates are called ¢. 
To the Plate & we 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 #&. 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 five radials are simple, and the Plate 2, 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 (Jocrinus, Fig. 9) were 
now parted, and the Plate x was introduced to fill the vacant space (Den- 
drocrinus, Fig. 13). These modifications, however, did not affect the position 
* Quart. Journ. Geol. Soc. London, Vol. XXXVIIL., pp. 298-312. 
+ Ann. and Mag. Nat. Hist. (6th Ser.), Vol. V., April, 1890, pp. 319 to 334. 
