1881.] NATURAL SCIENCES OF PHILADELPHIA. 18t 



or indirectly, connected with that organ. In the earlier genera, 

 Glyptocriniis, Beteocrinus, Glyptaster, Archaeocrinus, Eucrinus, 

 and in all genera up to the Subcarboniferous in which the anal 

 opening is lateral, the posterior area is distinctly' divided into two 

 equal parts by a single median row of plates. This row, which 

 often extends to the anal opening, is composed of the true anal 

 plates, but the plates on either side of it are interradials. By 

 considering the latter, without regard to the median row, it will 

 be found, that the two sections taken together correspond exactly 

 in number and general arrangement with the interradials of the 

 other areas, or at least differ not more than the other four differ 

 among themselves. But it must be observed that in genera in 

 which the first anal plate rests directly upon the basals {Glyptaster, 

 Eucrinus, Dorycrmus, etc.), the first true interradial in the pos- 

 terior area is divided, and is represented by two smaller plates, 

 separated by a special anal plate (PI. XIX, fig. 2). In these genera 

 the anals proper extend from the basals to the anal aperture. 

 Glyptocrinus and its congeners, in which there is no anal piece 

 represented between the first radials and in which the first interra- 

 dial range, consists of a single plate in all five spaces, the special 

 anal plates begin with the second range in which there are three 

 plates. In later geological times, when the anal opening became 

 more central, the special anal plates decreased to two or three, 

 and in the t3'pical Actinocrinidie in which we include the genera 

 Actinocrinus, Amphoracrinus, Strotocrinus, Physetocrinus and 

 Steganocrinus, they are reduced to a single plate. 



Some of the Platj^crinidas have no special anal piece, and the 

 posterior side differs merely b}^ having a somewhat larger inter- 

 radial, others however, as Dichocrinus and Hexacrinus, have a 

 very large special plate. In some of the Rhodocrinidfe, like the 

 Calyptocrinidfe, the symmetry of the cal3'x is almost regularly 

 pentahedral, and none of the plates of the posterior side are 

 actually anals. 



4. Interaxillary Plates. 



The space within the axil of the secondar}^ radials is frequently 

 filled by plates, for which we proposed in the first part of this 

 work the name " axillarj^ plates." This designation is undoutedly 

 appropriate, but finding that it had been previousl}- used b}' several 

 authors for the bifurcating plates, we have thought best, in order 



