2460 . The American Naturalist. [March, 
of the region, which gives every reason to believe that the 
changes went on quietly, yet at a rather rapid rate. The great 
abundance of individuals at this time may be due, in part at least, 
to the withdrawal of their more motile enemies because of the 
unsuitable physical impositions already mentioned. The com- 
paratively rapid changes of environment thus imposed would 
force rapid modifications in the structure of the various individ- 
uals in order to secure a more. perfect adaptation to the new 
conditions. And when these physical changes went on with 
still greater rapidity structural adjustment was unable to keep 
pace, and soon ended in the extinction of the group. The un- 
favorable conditions at a somewhat later period are further shown 
in the neighboring districts where a few types still persisted, 
small, depauperate and few in numbers. 
Generic Considerations—The Actinocrinoids are first known 
in the upper Silurian. They early showed signs of departure 
from the primitive form ; and developed chiefly along two diver- 
gent lines. The one group continued to the Burlington with but 
slight tendencies to modification in general structure ; the other 
soon broke up into a number of more or less well-marked sec- 
tions, each of which rapidly expanded into new generic types, 
until about the close of the Keokuk, where, with a single excep- 
tion, they became extinct. The present account will therefore 
make mention of the following groups as comprising the Actino- 
crinide: Periechocrinus, Megistocrinus, Amphoracrinus, Al- 
loprosallocrinus, Agaricocrinus, Dorycrinus, Gennzocrinus, Eret- 
mocrinus, Batocrinus, Actinocrinus, Teleiocrinus, Physetocrinus, 
Strotocrinus and Steganocrinus. 
The general structure of the forms has already been alluded to, 
but some minor anatomical points in various genera may require 
further consideration. The first of the sections above referred to 
includes only two types—Periechocrinus and Megistocrinus. 
These genera differ from the other members of the family chiefly 
in the relatively large calyx, rather small branching arms, the 
large number of interradial plates, and in the structure of. the 
ventral surface. In Periechocrinus the plates are smooth and 
thin; in Megistocrinus rather thick and more or less highly or- 
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