242 PROCEEDINGS OF THE ACADEMY OF [1885. 



Journ. of Sci., 18f»9, vol. xxix, p. 79), and in another specimen 

 we found, inter radially disposed, small imbricating plates con- 

 uecting with larger pieces. Whether the latter, as we supposed, 

 represent the summit plates, or Lyon and Casseday's alternating 

 pieces, we could not make out satisfactorily. Carpenter took 

 them to be " covering plates of the ambulacra, which perhaps 

 \\ire permanently closed as in the Platycrinidae, or only tem- 

 porarily so as in the Neocrinoids ; while the small irregular 

 plates, which form the interradial portions of the vault, corres- 

 pond to the anambulacral plates of recent Crinoids. They pass 

 downward into the interradials at the sides of the calyx, just as 

 in the recent species and in the Liassic Exlracrinus" (Chall. 

 Hep., p. 181). We accept the first part of this explanation that 

 these alternate plates probably correspond to the covering 

 pieces of the Platycrinidae ; we even admit these plates to be 

 morphologically identical with those along the disk of the 

 Neocrinoidea. Bui we doubt if the interradial portions in 

 Onychocrinus, or Plati/crinus either, correspond to the anambu- 

 lacral plates of recent Crinoids. The interradial plates of vault 

 and disk are very distinct structures ; the former constitute a 

 part of the abactinal system, while those of the disk are actinal. 

 Before we enter upon further discussion of this subject, we direct 

 attention to the ventral structure of the Blastoidea and Cyatho- 

 crinidae. 



The Cj'athocrinida' were described by us as having no inter- 

 radials, and until lately we considered this a fixed character of 

 this group. The fact that the only plates interradial in position 

 are located ventrally, seemed to us as sufficient evidence that 

 they were actinal plates, and as such they seemed to be the 

 representatives of the oral plates in the Neocrinoidea. We 

 thought the same regarding the deltoids in the Blastoidea, which 

 occupy essentially the same position in relation to adjacent parts 

 .■is the above plates in the Cyathocrinidae. Prof. Zittel, in his 

 " Handbuch der Palseontologie, i," like us, called the plates orals 

 in all three groups, and this interpretation was afterwards 

 accepted by Mr. Etheridge, Jr., and P. Herb. Carpenter, in 

 their paper, "On certain points in the Morphology of the 

 Blastoids " (Ann. Mag. Nat. Eist., April, 1882, p. 214), in which 

 these writers state thai in Blastoids the calyx is formed "by 



