98 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



Neumajr's interpretation of the plates meets with serious difficulties. 

 There are in Cyathocrinus undoubtedly two sets of plates ; the one occupy- 

 ing the centre of the disk^ and covering completely mouth and peristome, 

 without grooves, and with the ambulacra subtegminal ) the other occupying 

 the outer margins of the disk, grooved, the ambulacra tegminal, and covered 

 over by perisome which extends inward and closes the mouth. He under- 

 took to explain these difficulties by palaeontological development, but over- 

 looked the fact that in the ontogeny of recent Crinoids the perisome is 

 introduced above the radials, and between the orals, and that the latter are 

 carried relatively inward. The same mode of development we find in the 

 phylogeny of fossil forms ; the orals, with the introduction of interradial 

 plates, are moved to the centre of the disk, and either cover the mouth 

 or immediately surround it. That is the case in the Camerata and the Articu- 

 late Ichthyocrinidae, and there is no reason to doubt that it is the same way 

 in the Cyathocrinidse. In the Larviformia, however, in which there is no 

 perisome, the orals rest against the radials, but also cover the mouth, as 

 they do in the Pentacrinoid larva. 



We beheve with Neumayr that the differentiations among the species 

 which we have noticed in the disk of Cyathocrinus are modifications due to 

 palaeontological development. It seems to us that the orals throughout this 

 genus are more or less in a state of resorption, more advanced in one species 

 than in another, and even varying in degree in the same species. From this 

 we conclude that the Silurian C. alutaceus, in which the orals are almost or 

 wholly intact, represents the more primitive form of the genus, and C. malva- 

 ceus, etc., a later stage ; and that Euspirocrinus, in which the orals are appar- 

 ently completely removed, and the ambulacra thereby brought into view 

 upon the disk, represents a more advanced stage than either species of 

 Cyathocrinus. 



For proof that a resorption took place in the same species, we refer to the 

 specimen of C. alutaceus, Fig. 7, which differs essentially from Fig. 6. It is 

 also proved by numerous specimens in our collection,* which show distinctly 

 that the orals are proportionally larger, and more regular in their arrange- 

 ment in young specimens than in the adult. In one of the specimens, not 

 larger than a good-sized pea, they occupy fully two-thirds of the disk, being 



^ * We liave from sixty to seventy specimens in most excellent preservation, representing jGve species, in 

 whicli we exposed the disk by removing the arms. Most of them came from Indian creeC Ind. (Keokuk 

 group), though some are from Burlington, and a few from Crawfordsville. 



