SYSTEMATIC PART. 163 
in the sense that Actinocrinus is a more advanced type than Plalycrinus. The 
two former represent the larval state of the Camerata, while Platycrinus is 
a sort of transition form, in which the Camerate stage has not reached its 
full development. 
The Camerata existed at the beginning of the Silurian, and survived 
to the close of the Subcarboniferous, with a feeble reminiscence in the Coal 
Measures. But although they developed some very remarkable and short- 
lived forms in the Silurian — such as the Calyptocrinide and Crotalocrinidx 
—the type is pre-eminently a Subcarboniferous one. In that age they 
reached an extraordinary development, not only in the abundance with 
which they flourished, both as to numbers and variety, but also in extrava- 
gance of form and size in every one of their leading families. In the lower 
Carboniferous the Camerate type seems to have achieved the summit of 
its possibilities, for extinction followed rapidly after, and at the close of the 
Keokuk epoch there was scarcely a remnant of the typical section left, and 
at the end of the Kaskaskia the whole group, so far as Paleontology informs 
us, was practically extinct. 
The Camerata fall naturally into two sections : — 
I. Those in which the lower brachials and interradials form an important 
part of the dorsal cup. 
II. Those in which the brachials retain the form and small size of arm 
plates, and the interradials are almost exclusively confined to the tegmen. 
The first of these represents the typical Camerata, of which an Actinocrinus 
is a characteristic example. It includes the Reteocrinidse, Rhodocrinide, Thy- 
sanocrinids, Melocrinid, Calyptocrinide, Batocrinide, and Actinocrinide., 
This section reached its culmination among the Actinocrinide in the genus 
Strotocrinus, of which in some species the rays are incorporated as high as the 
twelfth order of brachials. 
The second, or non-typical, section represents a stage in which the modi- 
fication of the Inadunate type by Camerate tendencies only progressed to 
a limited extent, as shown by Platycrinus and allied forms. It includes the 
Platycrinide, Hexacrinide, Acrocrinide, and Crotalocrinide. In this section 
the Camerate type was not perfectly attained, but its development was 
checked. This may have been due to the large size of the radials, and the 
comparatively small size of the succeeding brachials, which retained perma- 
nently the condition of free arm plates. The species of this group are inter- 
mediate between the Inadunata and the typical Camerata, their lower arm 
