lying between them, the northern extension of the species 
peculiar to the one, and the southern range of those char- 
acteristic of the other are wholly unknown, and any attempt 
to discuss the reason underlying their distribution is neces- 
sarily futile when the facts of distribution themselves are 
wholly conjectural. 
All that can be done now is to state such facts as to the 
general distribution of the fauna of the state as are shown 
by the returns of the census, noting any apparent peculiart- 
ties of the range of the different species, and leave any syste- 
matic discussion of the subject to such future time as our 
increased knowledge will justify the attempt. 
Of the ultimate origin of our fauna, but little is known. 
The geologists tell us that the terrestrial mollusca range 
back in time certainly as far as the Carboniferous age and 
possibly into the Devonian. Indeed, the fact ‘‘that their 
diversity of form gives sufficient indication that the /lelicida 
had become widely differentiated during those early epochs 
in which they liv ed, probably quite as widely as their living 
representatives are, and under closely similar forms*” would 
indicate that their separation from their fluviatile or marine 
ancestors must have occurred at a much earlier date. 
A very large part of our fauna is peculiar to North 
America, and has undoubtedly descended from those ancient 
forms, which peopled the shores of the great Mezozoic sea and 
hid under the bark of the fallen eiants of the Carboniferous 
forests. Whether these early mollusca had spread into the 
Michigan of that day is not known. It seems entirely prob- 
able, but there is as yet no evidence either to prove or dis- 
prove the existence of such a fauna in this region prior to the 
Glacial epoch. 
But, however that may be, the immediate source of our 
present fauna must be sought in the states lying to the south 
and beyond the reach of the ice sheet, which in the Glacial 
period buried Michigan under hundreds of feet of ice and 
utterly exterminated every form of molluscan life that may 
have previously existed here. The extensive Post-pleiocene 
deposits in the Mississippi valley prove conclusively that the 
fauna then existing was substantially the same as is now found 
in that region. With the retreat of the ice, the mollusca 
returned to the north and repeopled the new land. It seems 
probable that even the so-called circumpolar species, which 
probably originated in the old world, had made their advent 
into America prior to the Glacial period, during some earlier 
age, when a milder climate in the extreme north was more 
* White, Review Non-Marine Fossil Moilusca of N. A., p. 445. 
ie) 
