SYSTEMATIC PART. 163 



in the sense that Actmoermus is a more advanced type than Flatycrimis, 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 Calyptocrinidse and Crotalocrinidse 

 — 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 flimilies. 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 Palaeontology 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 Adinocrinus 

 is a characteristic example. It includes the Eeteocrinidas, Rhodocrinidae, Thy- 

 sanocrinidae, Melocrinid^, CalyptocrinidaB, Batocrinidse, and Actinocrinidse. 

 This section reached its culmination among the Actinocrinidse 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 Plalycrimis and allied forms. It includes the 

 Platycrinidae, Hexacrinidae, Acrocrinid^, and Crotalocrinidae. 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 



