1890.] NATURAL SCIKXCKS OF I'lIILADKLrillA. 357 



reception of the ambuhicra. In another specimen of the same species, 

 fig. 6, on tlie same phite, four of tlie " interradial plates " are almost 

 completely covered by minute, very delicate perisomic i)ieces, but of 

 the posterior one the greater part of the surface is bare. In C'yatho- 

 crinus ioivoisls, PI. X, figs. 2-3, the larger plates are so closely united 

 that it appears as if they formed a continuous undivided ring around 

 the peristome, and served as a support for the delicate perisomic plates 

 on top. This was the opinion of Wachsmuth, who in 1877 (Amer. 

 Journ. Sc, Vol. XIV, pp. 183 and 184), regarded them as constitu- 

 ting asort of consolidating apparatus like that described by Roemer in 

 C'upressocrinus. We afterwards (Revision Pt. I, p. 12), suggested 

 that the " consolidating plates " of Cyathoeriuns, and those of Cupres- 

 socrinus, were structurally identical with the deltoids of the Blastoidea, 

 and both homologous with the orals in the Antedon larva. Similar 

 views were expressed by Prof. Zittel, Dr. Carpenter, and lately by Dr. 

 Neumayr who all agreed that those plates in Cyathocrinus were orals. 

 This interpretation, which at first seemed most plausible was abandoned 

 by us in 1884, and also by Carpenter, owing to a morphological diffi- 

 culty which it involved ; for the ambulacra would then have to pass 

 ovei; and not betiveen the edges of the plates, a combination which 

 seemed to us at variance with the nature of the oi'als. Since then, 

 until lately, we have regarded these ])lates as interradials ; but with 

 considerable hesitation, for the plates are neither interradial nor inter- 

 ambulacral, but for the greater part su^-ambulacral. In their rela- 

 tions to surrounding parts they differ essentially from the interradials 

 of Platycrhms or those of any other Camerate genus. Besides in 

 Platycrinus the ambulacra rest against the edges of the interradials, 

 and only the covering pieces are exposed on the surface ; while in most 

 of the Cyathocrinidae, if not in all, the whole ambulacrum rests on top 

 of them, and the small perisomic plates sustain toward the side and 

 covering pieces the same relations as the interradial plates of 

 Flatycrinus. That the plates are not orals, is further proved by the 

 fact that there are in Cyathocrinus iowensis other large plates cover- 

 ing the peristome, which naturally represent them. The orals, 

 which in C. alutaceus are unchanged through life, apparently were 

 wholly or partly resorbed in other species, and their places occupied 

 by large covering plates, of which the proximal ones joined in the 

 center. This is well shown in the specimens PL IX, figs. 5, 7, 8, 9, 

 and PI. X, figs. 1-3, and seems to have been the case in Cyathocrbms 

 laevis PI. IX, figs. 2 and 3, and Euspirocriuus spiralis, PI. IX, figs. 



