98 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
Neumayr’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 paleontological 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 Ichthyocrinide, and there is no reason to doubt that it is the same way 
in the Cyathocrinide. 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 believe with Neumayr that the differentiations among the species 
which we have noticed in the disk of Cyathocrinus are modifications due to 
paleontological 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. adutaceus, in which the orals are almost or 
wholly intact, represents the more primitive form of the genus, and C. madva- 
ceus, etc., a later stage; and that Luspiroerinus, 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 have from sixty to seventy specimens in most excellent preservation, representing five species, in 
which we exposed the disk by removing the arms. Most of them came from Indian creek, Ind. (Keokuk 
group), though some are from Burlington, and a few from Crawfordsville. 
