3 University of Michigan 



The first emigrants from the West to the headwaters of 

 the IMesozoic rivers, in the Laurentian and Appalachian High- 

 lands, through orographic changes which caused the transfer- 

 ence of streams, were undoubtedly able to penetrate in one 

 way and another into the early streams which flowed easterly 

 and southeasterly into the Atlantic Ocean, but ultimately the 

 continued rise of the Appalachian System resulted in the en- 

 tire separation of the eastern and western faunas and enabled 

 each of them to develop under local influences, wholly inde- 

 pendent of each other. 



In the same way changes in the drainage systems of the 

 rivers from the highlands of Tennessee, which originally had 

 flowed southerly into the Gulf of Mexico, by which some of 

 them — those which are now comprised in the Cumberland and 

 Tennessee systems of drainage, — were cut off from their south- 

 ern connections and established a new system of drainage to- 

 ward the northwest into Ohio, resulted in the separation of 

 the fauna in the streams south of the Tennessee Highlands 

 from that inhabiting the streams which flowed westerly through 

 what is now the Tennessee and Cumberland Systems, and 

 enabled these, also, to develop under their own peculiar local 

 influences. 



Such was the situation at the beginning of the Glacial 

 Epoch. This covered with ice to the depth of several thou- 

 sand feet practically all the territory in eastern North Amer- 

 ica north of the Ohio River. The result was, so far as the 

 Unionidge are concerned, the absolute extermination of the 

 whole race north of the line of glaciation. This wiped out all 

 the connecting links, which might have then existed between 

 the eastern, or Atlantic, fauna, and the western, or Missis- 

 sippi, fauna, and left them, as represented along the southern 

 borders of the glaciated areas, as distinct faimas, and from 



