MOLLUSCA ;0F MICHIGAN— WALKER. 445 



fauna of nineteen, or nearly one-third, is entirely unknown; twenty-one are 

 represented by less than ten species; eleven by from ten to twenty species, and 

 only seventeen by more than twenty, nine by more than thirty, two by 

 more than forty, and one by more than fifty. Kent is the banner county 

 with sixty species to its credit. When it is considered that all of the south- 

 ern counties have, in all probability, as large a fauna as is now reported from 

 Kent, and that the species reported from Charlevoix and Chippewa couijties 

 range through the entire northern part of the lower peninsula, the poverty of 

 our knowledge and its entire inadequacy for anything like positive state- 

 ments in regard to the distribution of the different species is only too obvious. 



The great extent of unknown territory in the center of the state north of 

 the Saginaw-Grand valley is especially noticeable and is particularly un- 

 fortunate. The Saginaw-Grand valley and the counties lying south of it, 

 and the Grand Traverse region, have been sufficiently explored to give a 

 substantially accurate knowledge of their fauna as a whole. But with this 

 great stretch of unexplored territory lying between them, the northern ex- 

 tension of the species peculiar to the one, and the southern range of those 

 characteristic of the other are wholly unknown, and any attempt to discuss 

 the reason underlying their distribution is necessarily 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 distribu- 

 tion of the fauna of the state as are shown by the returns of 'the census (Plate 

 II), noting any apparent peculiarities of the range of the different species 

 and leave an}^ systematic 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 

 "ihsit their diversity of form gives sufficient indication that the Helicidce 

 had become widely differentiated during those early epochs in which they 

 lived, 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 un- 

 doubtedly descended from those ancient forms, which peopled the shores of 

 the great Mezozoic sea and hid under the bark of the fallen giants of the 

 Carboniferous forests. Whether these early mollusca had spread into the 

 Michigan of that day is not known. It seems entirely probable, but there 

 is as yet no evidence either to prove or -disprove 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-pliocene deposits in the 

 Mississippi valley prove conclusively that the fauna then existing was sub- 

 stantially 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 ex- 



* White, Review Non-Marine Fossil Mollusea of N. A., p 445. 



