164 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



joints taking part in the composition of the calyx by means of plates whicli 

 combine the functions of interbrachials and interambulacrals, and which, 

 properly speaking, are plates of the disk. Another peculiarity characteristic 

 of this section is the presence of a large facet upon the radials, in which 

 the lower brachials are buried, and a brachial of higher rank meets the 

 interradials, and sometimes the radials. 



The question has been asked whether the Crotalocrinidge should not 

 be placed under the Inadunata, instead of the Camerata. They certainly 

 represent an intermediate form, having some characters even of the Articu- 

 lata. But their lower brachials are more or less connected with plates of 

 the calyx, and the covering plates of the ambulacra, unlike those of the 

 Inadunate Crinoids, are rigidly incorporated into the tegmen. They are 

 morphologically in the same condition as the other families of this section, 

 except for the dicyclic base, and represent, as we conceive, only a different 

 degree of departure from the Inadunate plan. 



The typical section of the Camerata appears to have been the first in 

 time. It was well defined in the Lower Silurian, where it was represented 

 both by dicyclic and monocyclic forms,— the Rhodocrinid^ on the one hand, 

 and the Batocrinid^ on the other. They flourished about the same period, 

 culminated together in the Burlington epoch, and disappeared almost simul- 

 taneously, the one in the Keokuk group, the other in the Warsaw limestone. 



In the Lower Silurian there was another family — the Reteocrinidse in 



which the structure of the base seems to have been subordinate to other 

 characters, and we found it advisable to include among them monocyclic and 

 dicychc forms; it was short-lived, not surviving the Hudson River group. 

 The ThysanocrinidaB and Calyptocrinidge, the former dicyclic, the latter 

 monocycHc, came to light in the Upper Silurian, with a very small beginning 

 for the former in the Hudson River; they existed for a time in considerable 

 abundance, but perished soon, only a few straggling forms surviving to the 

 Devonian. Of the monocyclic families, the Melocrinidse were the earhest, 

 ranging from the Trenton to the Hamilton, where they seem to have been 

 abruptly cut off. The other great monocyclic family, the Actinocrinidse, 

 appeared, culminated, and disappeared in the Subcarboniferous. 



The non-typical section made a good beginning in the Upper Silurian 

 with its only dicyclic family — the ephemeral Crotalocrinid^ — and the 

 Platycrinid^, represented by ^ve genera, of which four expired before the 

 close of the epoch. The surviving Platycrinid^ had a feeble representation 



