SYSTEMATIC PART. 165 
in the Devonian, but during the Subcarboniferous attained in the genus 
Platycrinus 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 
Hexacrinid 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 Acrocrinus Worthent. 
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 palzonto- 
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 Rhodocrinids 
were reduced to pointed hexagons in the Thysanocrinidx, 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 Rhodocrinide 
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 Gilbertsocrinus being their last survivor; while the Thysanocrinide 
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 Batocrinide and Hexacrinide. 
The pentagonal base, though reinforced in the Niagara by the Calypto- 
crinide, disappeared from the typical Camerata with the Melocrinide in the 
Hamilton; while the hexagonal base, with its accompanying anal plate, 
continued with great vigor in the Batocrinidex and their offshoot, the Actino- 
crinidx, 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 Crotalocrinide having a truncated posterior basal 
