1886.] NATURAL SCIENCES OP PHILADELPHIA. 153 



Family XXII. POTERIOCRINIDJE. 



It has been stated that the distinction between Poteriocrinidse 

 and Cyathocrinidae is based principally upon their different mode 

 of articulation. The articulation, however, in the Poteriocrinidse 

 undergoes certain modifications, and toward the close of the Car- 

 boniferous even the calj'x begins to show signs of being capable 

 of slight expansion, if not mobilit}^, the plates being frequently 

 provided with a fossa along their margins ; while in others the 

 plates are united by syzygy. Those fossae which extend over the 

 whole surface of the opposed faces not only occur between the 

 radials, but extend in a vertical direction to the basi-radial suture. 

 That they lodged bundles of ligament is almost beyond doubt, 

 and Dr. P. H. Carpenter, whom we consulted on this subject, 

 regards this union as a suture, but "less close, with longer liga- 

 ments, and therefore more expansible and movable, than in an 

 ordinary suture." A syzygial union is found among the species 

 which Trautschold referred to the genus Gromxjoci'inu:,^ and 

 seems to extend in some of them even to the suture between 

 basals and underbasals, the striation being plainly visible along 

 the suture lines. In some of these genera the articulation between 

 brachials and arm plates is as highly differentiated as in most of 

 the Neocrinoidea, and much more so than in some of them. 



The progressive development in the mode of articulation that 

 takes place from the earlier to the later Poteriocrinidae is, no 

 doubt, of some classificatory value, but we have not been able, 

 as yet, to separate them on this ground, as the respective parts 

 are generally in an imperfect state of preservation, and there 

 appear to be no other distinguishing characters. Moreover, a 

 comparison of the earlier Poteriocrinidae, such as Scytalocrinus, 

 Scaphiocrinus, Pachylocrinus, Cteliocrinus and Graphiocrinus^ 

 with the latter genera E upachycrimis, Zeacrinus^ Hydreionocri- 

 nus, Geriocrinus, Erisocrinus, and of the latter with Encrinus, 

 shows that the two sections shade into one another by such easy 

 gradations, that in many cases it is absolutely impossible to draw 

 a dividing line. 



The genera of the Poteriocrinidae are principall}^ based upon 

 the arrangement of the plates at the az3^gous side, the form of 

 the cal3'x, the proportionate size of the ventral sac, and the 

 branching of the arms. Some of the genera have a well-developed 

 azygous plate, a special anal piece, and one of the plates support- 



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