162 Cincinnati Society of Natural History. 
primitive barriers, but it remains, as we have seen, true to its original 
instincts in all its more important phases. It is not probable, as may 
be suggested by the doubting reader, that this fauna would have been 
exterminated by the great glacier, which is supposed to have origin- 
ated in its peculiar haunts, but more likely that the few species having 
an abnormal southern or southwestern range, received the first impulse 
of distribution in that direction from the glacial condition; and that 
with the northward retreat of the glacier they simply resumed their 
normal habitat, continuing their distribution in that direction in suc- 
ceeding times to the northern lakes of British America. In case of 
Fauna B we have evidence that a previous distribution, probably sev- 
ered, by the same or other causes, has never been fully united in a few 
cases, as in that of the M. margaritifera, occurring in Maine and Ore- 
gon, but not between these stations so far as now known. But in most 
cases, the re-union has been complete. Such remnants as the glacial 
epoch left, have been equal to the emergency of perpetuating their 
race over the region desolated by glacial action, and they may thus 
indicate what are the possibilities of development under determinate 
conditions. It may be suggested, that as the species of so-called 
Strepomatide of the west coast have rather the facies of the tropical 
Melanians; and as the other associates of the M@. margaritifera in the 
waters of Oregon are species not elsewhere found, that this little 
faunal remnant is an independent one, and I readily agree to all this ; 
yet there is no doubt of the existence of a Fauna B, nor of its distri- 
bution, and the possibility that its present species are the descendants 
of a geological remnant like those of A. Still more striking is the’ 
evidence to be adduced from Fauna ©, The region over which this 
group is distributed may have had some drainage, though perhaps 
slight, as far back as the epoch of the Cincinnati uplift. It thus may 
have continued through all the Palzozoic ages thereafter. What wonder, 
then, that we have here such a diversity of forms, when we remember 
the mutations through which the continent subsequently passed to the 
termination of the Paleozoic. Local elevations and submergencies, 
and the various phenomena associated with the progress of continental 
development, brought to these creatures a series of vicissitudes that 
may have left many remnants in favored spots, whose descendants, 
modified and changed as they are, afford us the multitudinous varieties 
which this fauna assumes throughout its metropolis. 
Indeed, if we could reach the ancestral form of these creatures, we 
should have another proof of the existence of what Prof. Dana so philo- 
