164 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
joints taking part in the composition of the calyx by means of plates which 
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 Crotalocrinide 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 Batocrinidx 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 Reteocrinidz — 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 
dicyclic forms; it was short-lived, not surviving the Hudson River group. 
The Thysanocrinide and Calyptocrinidse, the former dicyclic, the latter 
monocyclic, 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 Melocrinide were the earliest, 
ranging from the Trenton to the Hamilton, where they seem to have been 
abruptly cut off. The other great monocyclic family, the Actinocrinide, 
appeared, culminated, and disappeared in the Subcarboniferous. 
The non-typical section made a good beginning in the Upper Silurian 
with its only dicyclie family — the ephemeral Crotalocrinida — and the 
Platycrinide, represented by five genera, of which four expired before the 
close of the epoch. The surviving Platycrinidx had a feeble representation 
