PERISCHOECHINIDAE. 645 



sponding to those which we might find in the plates of the abactinal area. 

 From the moment the presence of teeth was clearly proved, and the exist- 

 ence of a large oral opening demonstrated, placed opposite an anal region, 

 surrounded by plates entirely homologous to those of the ovarian and ocular 

 plates of the Echini, their affinity to Crinoids could no longer be maintained ; 

 as we find nothing in any Crinoids thus far known which could in a remote 

 degree be homologized with those areas, as constructed in the group of Pa- 

 laechinidae. There remained, therefore, nothing but to place them among 

 Echinidae, and this view is now generally adopted ; the value of the charac- 

 ters of the group, whether ordinal or subordinal, being the only ones ques- 

 tioned. No writer thus far has as yet succeeded in homologizing these Echini 

 with our recent Echini ; the structure of the ambulacral and interambu- 

 lacral system finding no parallel apparently in any of the recent Echini. 

 Miiller, while establishing the genus Lepidocentrus,* took occasion, in the 

 course of his description of the isolated plates and spines which were found 

 in the Eifel, to show that it was possible for Echini to have such imbricating 

 scales as we find in Lepidocentrus, stating, however, at the same time, that 

 there exists nothing of the sort in the recent Echini ; the only irregularly 

 shaped plates known are those of the actinal system of Cidaris, with which, 

 however, he considers they have nothing in common. Miiller was the first 

 to draw attention to the value of this set of plates in distinguishing the 

 Cidaridae from the Echini, but he has not, I think, fully appreciated the 

 value of this part of the actinal system of the family. It is well known that 

 the Cidaridae and Echinothuriae have no cuts in the test for the passage of 

 the actinal gills, but we find these cuts directly at the point of contact of 

 the buccal membrane and the teeth, and there the {/ills make their appearance. 

 Miiller himself has drawn these cuts in his plate,t in the very memoir where he 

 denies the existence of the gills, and it is somewhat astonishing they should 

 have escaped the notice of such an admirable observer. It is well known 

 that in the Cidaridae we have only a small number of coronal plates ; in 

 the largest Cidaris I have ever seen (Phyllacanthus gigantea), measuring 2.5 

 inches in height and 3.5 in diameter, there are only ten. If we examine a 

 young Cidaris, we find the coronal plates reduced to a minimum, but, con- 

 trary to what is the case in young Echini (Strongylocentrotus) of the same 

 age, where no buccal membrane is developed,! we already find the buccal 



* Lepidocentrus of Muller, which has thus far escaped the attention of American Paleontologists, is 

 closely allied to Lepidechinus of Hall, if not identical with it. 

 f Muller, Ueber den Ban d. Echinodermen, PL IT. f. 7. 

 % A. Agassi z, Emb. of Echinod. in Mem. Am. Acad., Fig. 27. 



