132 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
See our diagram (Fig. 11). So also the plate ¢ of Heterocrinus bellevillensis 
(Fig. 10), though less symmetrically disposed, rests upon both radials, and 
not on the plate to the right only, as figured by Bather. 
We now pass to those forms in which the inferradial R’ performs the 
functions of an anal plate. The first step in this direction is shown by 
Homocrinus (Fig. 14), Botryoerinus, Oneoerinus, and Barycrinus, in which the 
superradial # has shifted slightly to the right, and is connected with the 
inferradial by an oblique suture; contrary to the case of Dendrocrinus, 
in which the two plates are arranged vertically (Fig. 13). The plate ¢ in 
those forms rests upon # without touching #’, and the plate z, which is 
comparatively large, rests against both sections of the compound radial. 
Bather explains the evolution that took place in these forms by “ the 
shifting of the radianal,” and that “z has sunk still lower into the dorsal 
cup, and is now on a line with the radials.” Nothing of this kind is indicated 
by the specimens, which clearly show that the radianal throughout the 
Fistulata retains the same position, whether it constitutes a part of the radial 
or serves as anal plate. It only changes its outlines so as to conform to the 
shape of contiguous plates. As the tube became larger, the radials spread 
out, and the vacant space thus formed was filled by a new plate, z. There 
was no sinking of the plate ¢, which never moved from its place above the 
radials. 
Another stage in the developmental history of the anal area among the 
Fistulata is presented by Parisocrinus, Atelestocrinus, Euspirocrinus, and the 
typical Poteriocrinidee. Bather, in alluding to them, makes the following 
statement: “In the Poteriocrinites (see our diagram Fig. 2) another change 
has taken place; the radianal has passed through a revolution of 90°, and the 
lowest plate of the ventral sac (¢) has sunk down between # and x.” If we 
understand what this means, he assumes that the lower section of the radial 
moved to the left; while in fact, as the specimens show, it was the upper 
section of the plate that moved away, shifting to the right and leaving 
a space for R’ and ¢ to meet. Thus it was that # came to rest against 2’, 
but not against 7, from which it was separated by the plate ¢. An increase 
of width in the anal area became necessary, as the surface for the support of 
the tube was insufficient to hold it. In Homocrinus and Dendrocrinus, the 
costals occupy only a comparatively small part of the radial, and a rather 
large portion of the latter serves as a support for the tube. In the Poterio- 
crinide, however, and in Parisocrinus, Luspirocrinus, and Atelestocrinus, in 
