278 PROCEEDINGS OF THE ACADEMY OP [1885. 



ring of orals, yet all of this must be encountered if we con- 

 sider the proximals to be the orals. Moreover, in Strotocrinus 

 and Teleiocrinus the two posterior radials would be placed 

 inside the oral ring, the orals of Megistocrinus would enclose a 

 large circlet of interradial pieces ; while in Beteocrinus, Glypto- 

 crinus and other Silurian genera, the orals would be altogether 

 unrepresented. All these difficulties are removed if we regard 

 the central piece as the representative of the oral pyramid, and 

 the proximals as summit interradials. Basals and radials, inter- 

 radials and anal plates are then found to occupy the same position 

 orally as aborally, and even the small intercalated pieces in the 

 dome of Megistocrinus are explained by analogous plates in 

 the catyx of Acrocrinus. But on the contraiy, if the proximals 

 were the orals, it would follow that the orals were represented in 

 the calyx by the proximal interradials, and not by the basals. 



That the proximals, which are such prominent plates in the 

 Palseocrinoidea, are unrepresented in the Neocrinoids, is fully 

 explained by the fact that in the latter the interradials generall}' 

 were imperfectly developed in the catyx, and hence their absence 

 in that group cannot be considered a serious objection to our 

 views. 



We are convinced that neither the underbasals nor the dorso- 

 central are represented at the summit, especially not the latter. 

 We cannot imagine what office such a plate could possibly have 

 had at the oral side, considering that it constitutes a part of the 

 column, and the Echinoderms at no time, or in any group, were 

 attached at their oral side. That it is represented dorsally in the 

 Stellerids and Urchins is natural, as it represents there in a 

 wider sense the entire column of the Crinoid, but its presence at 

 the oral side would be an anomaly. 



It seems to us that a far less objectionable explanation of the 

 central plate than that given by Carpenter, would be to regard it 

 as a posterior oral. In this case the orals would he represented 

 by five plates and not by six ; the anus woidd be placed outside 

 the oral ring, and the radial dome plates would occupy the same 

 position towards the orals as the calyx radials toward the basals. 

 But it would place the mouth underneath the posterior oral, and 

 it offers no explanation of the central piece in Eaplocrinus. 



This view was, perhaps, taken by Zittel in the case of the 

 summit plates of Crotalocrinus and Enallocrinus, in which the 



