10 



While there is no doubt that future investigation will 

 extend the range of some of the species, and thus to a certain 

 extent, perhaps, modi f 3* the conclusions based on our present 

 information, 3-et, in view of all the facts, I cannot but feel 

 that the3 T are substantially correct, although of course they 

 must be considered rather tentative than final, and are pre- 

 sented now simply as embod3'ing the present state of our 

 knowledge on the subject. 



If then, these facts are true, the question at once arises, 

 as to what explanation can be found for them. 



As has been stated, there are at the present time no 

 natural barriers which limit the range of an3 T of the species 

 north or south. 



The comparatively slight differences in temperature can- 

 not be considered as a factor, because man3 T of the species 

 which are limited apparently to southern Michigan range 

 north up the Mississippi valley into Minnesota and through 

 the valle3 r of the Red River of the North and similar streams 

 into Manitoba, and even farther north, and the far greater 

 extremes of temperature occurring in those regions do not 

 appear to have an3* effect upon their size and vigor. 



So far as known, the existing geological formations of 

 the state are not such as to have any influence upon the 

 chemical constituents of the water of the different rivers, 

 which would effect the mollusca inhabiting them. 



There being, then, no apparent solution of the problem 

 to be obtained from existing conditions, it becomes necessary 

 to consider the constituents of our present fauna, its origin 

 and history and, in connection with them, those great geolog- 

 ical and topographical changes in past ages which have re- 

 sulted in the state as it now exists. 



The eastern part of the United States in respect to its 

 unione fauna is divided into two areas or provinces, known as 

 Atlantic Region and Mississippi Region, the dividing line 

 south of the St. Lawrence river being the Appalachian 

 Mountains. Towards the north, however, some of the species 

 of the Atlantic fauna have an extended western range 

 through the great lakes and in British North America. In 

 the same wa3 r many of the species characteristic of the Miss- 

 issippi valley have extended north and east into Manitoba 

 and the St. Lawrence valle3 T . 



In Canada and throughout the lake region, we have, then, 

 a commingling of the two faunas, varying in proportion in 

 different places. 



In Michigan, the fauna taken as a whole, is almost en- 



