MORPHOLOGICAL PART. 125 
It thus appears that the anal plates vary considerably in their position 
and distribution, and in some groups are absent altogether. As a general 
rule, they are largely represented in species with a strong tube, or a protrud- 
ing lateral opening; while in forms in which the anus is central, as in the 
Kucalyptocrinide, or comparatively small, they are either wanting or but 
feebly represented. The anus, although more or less influencing the whole 
posterior area, did not necessarily require the introduction of anal plates, 
and when the tube was small, an enlargement of the regular interbrachial 
plate sufficiently increased the width of the area. The anal plates, therefore, 
do not appear to constitute an essential element of the Crinoids, but seem 
to be supplementary pieces, introduced when the space was insufficient to 
accommodate the tube. 
Among the Fistulata, the term “anal plates” has been applied to two 
plates of different origin, the one radial, and the other interradial. The 
latter is the homologue of the “ special” or “first” anal plate of the Came- 
rata, and rests upon the truncated posterior basal. When there are two 
plates in the species, as in most of the Poteriocrinidx, and occasionally in 
other families, the second — which is actually the first or lowest in point 
of position — is located obliquely to the right of the first, so as to encroach 
more or less upon the proximal face of the right posterior radial. Its lower 
angle rests upon the upper sloping faces of the adjoining basal, its wpper 
face supporting the first plate of the tube, which in some of these genera is 
partly enclosed in the calyx. To understand the position of the lower plate, 
it should be noted that throughout the modifications which took place in 
the posterior interradius in geological time, this plate always retained its 
alternate arrangement with the basals, and occupied a radial position from 
the beginning. Its form was changed, and to some extent its relation to 
surrounding plates, by the increasing width of the ventral sac, which caused 
a displacement of the arm-bearing plates. The oblique position which the 
plate holds toward the posterior basal and the regular anal plate, and the 
latter toward the first plate of the tube, gives to these pieces a sort of 
alternate arrangement, which is continued throughout the tube. 
The changes that took place among the anal plates in the various groups 
of the Fistulata have been discussed by us at different times,* from which it 
appears our views have undergone considerable modification as the result of 
* 1879, Revision, Part I., pp. 71 and 72; 1883, Amer. Journ. Sci., Vol. XXVI., pp. 365 to 377; 1885, 
Rey. Pt. III., Sect. I., pp. 11, 12, 40; and 1886, ibid., Sect. II., pp. 196 and 210. 
