37(i PRO('i:RDrN(;s of thk academy of [1890. 



crinidae, like Cyathocrinus, have but one large anal ])late resting 

 upon the basals. In the Eucalyptocrinidae there are no anal plates 

 in the dorsal cup, and the five interradial areas are perfectly sym- 

 metrical. The same is the case in Dolatocrinus, Stereocrinus, Cen- 

 trocrinus, Allocrinus and Patelliocriniis ; while the typical Melo- 

 crinidae have an anal plate in one or more of the upper rows. 

 In the Rhodocrinidae the anal plates are in a similar condition as 

 in the Melocrinidae, their posterior area being undisturbed in some 

 of them, while in others a few anal plates are introduced ; but the 

 Rhodocrinidae have at all five sides a plate interposed between the 

 radials, which rests upon the basals. The Platy crinidae have no 

 special anal piece, but the middle plate of the first row at the poste- 

 rior side is considerably larger, and obviously served throughout 

 this group as anal plate. 



It appears from these observations that the distribution and posi- 

 tion of the anal plates varies considerably among the families, and 

 that in some of them they are absent altogether. As a general rule, 

 the anal plates are well represented in species with a strong tube or 

 a protruding lateral opening ; while in forms in which the anus is 

 central or comparatively small, as in the Eucalyptocrinidae, they 

 are either wanting or but feebly represented. The interposition of 

 the anus affected more or less the whole posterior area. In species 

 with a large anal tube, the increase in wudth thereby produced neces- 

 sitated the introduction of special anal plates ; while when the tube 

 was small, a mere enlargement of the regular interbrachial plates 

 sufficiently increased the width of the area. This shows that the 

 anal plates of the Camerata do not constitute a primary element, 

 but are supplementary pieces, which were introduced as the case 

 required. This is important as throwing light upon the anal plates 

 in the other groups. 



In the Fistulata the anals are the only plates interposed between 

 the rays. They consist of one or two pieces, or are unrepresented 

 altogether, and the ventral sac is supported by the radials. In spec- 

 ies with but one anal plate, this rests upon the posterior basal, 

 which is truncated for its reception, and hence is interradial in posi- 

 tion. When there are two plates, as in most of the Poteriocrinidae, 

 and in a few Cyathocrinidae, the second, which is actually the first 

 or lowest in point of position, is placed obliquely to the right of the 

 first, so as to encroach more or less upon the lower face of the right 

 posterior radial, its lower angle resting between the upper sloping 



