234 The American Geolop-isf. o-'toiHM-, 1901 
and in l)r()a(ler zones, and tbcrc developed their numerous 
specific and generic variations. 
But assuming a great age for this development, the ex- 
pression of suddenness is not unwarranted in referring to 
them. There is certainly slender suggestion in the Devonian 
of such large and opulent supplies of crinoidal life. The "bi- 
ological crisis" they present is not simply apparent. It is 
real. 
The crinoidal basins of the Lower Carboniferous illustrate 
again what is remarkable in the trilobites of the Cambrian, 
and the lamellibranchs of the Devonian, a great development 
of species in limited geographical areas. It is not one species 
from one locality, another from a second, and so on, Ijut a 
convergence of species at one locality or province. 
Amongst the possible elements of encouragement for the 
luxurious growth of crinoids in the waters of the Lower Car- 
boniferous seas, was the clearness of the latter, and their 
greater supply of lime than in the waters of our present seas. 
The withdrawal of the crinoids today into abyssal regions, 
while not universal, is a prevalent tendency, and may have 
some reference to the increased sedimentation along coast 
lines. It may also be inferred that this abyssal retreat is not 
congenial to a flourishing life of crinoids, and their disap- 
pearance from our modern seas is certain. 
The elevation of the Lower Carboniferous continent was 
probably low, the drainage to the coasts slow or inconsider- 
able, and it seems reasonable to suppose that the areas of very 
clear water approached the shores closer than they do today, 
offering an advantageous nidus for the development of crin- 
oidal life. The successful planting of the ova when the stalk 
commences to form (pentacrinoidal stage) may be attended 
with dangers to the life of the young crinoid, which .only 
clear water and peculiar bottom conditions can avert. As to 
the more rapid and easy assimilation of lime, the weight of 
the suggestion th'at the Lower Carboniferous seas contained 
more lime than those of today may be very slight. Yet it is 
cjuite easy to see that there is a difference in the assimilating 
functions of a Neocrinoid with its leathery tegminal crown, 
and that of a Palseocrinoid with its strongly plated calcareous 
dome, and that difference may have been brought about by 
some gradual decrease in the lime contents of the ocean. 
(To be Continued.) 
