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 latQral opening; while in forms in which the anus is central, as in the 

 Eucalyptocrinidae, 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 Poteriocrinidse, 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 upper 

 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 tlie 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 L, pp. 71 and 72 ; 1883, Amer. Journ. Sci., Yol. XXVI., pp. 365 to 377; 1885, 

 Rev. Pt. III., Sect. I., pp. 11, 12, 40; and 1886, ibid., Sect. IL, pp. 196 and 210. 



