58 THE NAUTILUS. 



only alternative is that the present Lake Erie fauna was derived 

 from an immigration of the typical forms from the south, and that 

 where these immigrants obtained a foothold in the interior waters of 

 the State, where the local conditions were substantially the same as 

 those in the Ohio Valley, they retained their characteristic form, 

 while such individuals of this invasion as remained in the Great 

 Lakes and were subject to the peculiar influences of that environ- 

 ment, became modified by it with the result, as shown by the present 

 conditions, of a varietal, but not a specific, differentiation. 



In conclusion, the deductions that would seem to naturally result 

 from the foregoing discussion are these: 



1. That the Atlantic fauna originated from a very early pre-glacial 

 invasion from the west, probably in late cretaceous or early tertiary 

 times. 



2. That the present extension of the Atlantic fauna towards the 

 northwest was the result of an invasion from the west, in post-glacial 

 times, most probably through the Mohawk and Trent outlets into 

 Georgian Bay, and from thence into Lake Superior. 



3. That the present existence of so large a representation of the 

 Mississippian fauna in Lake Erie is to be ascribed to a post-glacial 

 invasion from the Mississippi Valley through the Maumee outlet into 

 the post-glacial Lake Maumee. 



4. That the original pre-glacial fauna of the present St. Lawrence 

 system was absolutely exterminated during the glacial period, and 

 that the peculiar fauna now characteristic of Lake Erie is the result 

 of the modification from environmental causes of the post-glacial 

 immigrants from the south, and not the result of any survival in that 

 region of any part of the pre-glacial fauna. 



NOTE By an error on the part of the type-writer, Ptychobranchus 



phaseolus Hild. was omitted from the list of the Lake Erie species 

 on p. 22. It was included in the original draft of the paper and is 

 necessary to complete the tally of " thirty species " peculiar to that 

 lake mentioned on p. 23. It is an abundant species at the western 

 end of the lake, but dwarfed like most of the fauna. 



BIBLIOGRAPHY. 



1. Branner, John C. The Former Extension of the Appalachians 

 across Mississippi, Louisiana and Texas. Am. Jour. Sci. (4), iv, 

 1897, p. 357. 



