SYSTEMATIC PART. 165 



in the Devonian, but during the Subcarboniferous attained in the genus 

 Flatycrinus a wealth of forms which had no equal before or afterwards in any 

 other group. The struggle for existence was kept up in this section by the 

 Hexacrinidae long after the last typical Camerate Crinoid had disappeared, 

 and the expiring effort of an exhausted type is seen in the Coal Measures in 

 the form of the diminutive Acrocrimis Wortheni. 



It appears, therefore, that the typical Camerata do not represent the last 

 of them in point of time, but that either their final efforts at perpetuation 

 were carried on in connection with a tendency to revert to the Inadunate 

 type, or the greatest persistence was manifested by that form of the Came- 

 rata which had departed from it the least. 



The change from the pentamerous to the bilateral symmetry consequent 

 upon the introduction of an anal plate into the ring of the radials, was per- 

 haps the most important modification that took place during the palseonto- 

 logical history of the Camerata, and it occurred within the range of our 

 knowledge of the group. The symmetry of the dorsal cup, which through- 

 out the Trenton group had been more or less perfectly pentamerous, was 

 disturbed in the Hudson River group, in both dicyclic and monocyclic forms. 

 In the former, four of the truncated, heptagonal basals of the Rhodocrinidse 

 were reduced to pointed hexagons in the Thysanocrinidge, and the inter- 

 radial plates separating the radials disappeared from four sides, that at the 

 fifth retaining its position, and serving as an anal plate. The Rhodocrinidse 

 were a long-lived family, appearing in the oldest Silurian, and persisting to 

 the climax of the Camerata in the Subcarboniferous, — the strange, extrava- 

 gant Gilhertsocrinus being their last survivor; while the Thysanocrinidge 

 scarcely survived the Silurian. 



Among monocyclic forms the disturbance of symmetry was caused by the 

 interposition of an anal plate between the posterior radials, which converted 

 their pentagonal base into the hexagon of the BatocrinidaB and Hexacrinidse. 

 The pentagonal base, though reinforced in the Niagara by the Calypto- 

 crinidae, disappeared from the typical Camerata with the Melocrinidse in the 

 Hamilton; while the hexagonal base, with its accompanjnng anal plate, 

 continued with great vigor in the BatocrinidaB and their offshoot, the Actino- 

 crinidae, throughout the period of greatest development of the group, and 

 until the extinction of the typical section in the Subcarboniferous. 



In the non-typical section, we have among dicyclic forms no example of 

 a symmetrical base, the Crotalocrinidae having a truncated posterior basal 



