THE NAUTILUS. 81 



The discussion, then, includes the consideration of four questions: 

 1st, the origin of the Atlantic and Mississippian faunas; 2d, how and 

 when the peculiar extension of the Atlantic fauna to the northwest 

 took place; 3d, how and when the extension of the Mississippian 

 fauna into the Great Lakes took place; and 4th, whether the pecu- 

 liarly modified fauna of Lake Erie, as it exists to-day, is the result 

 of a pre-glacial invasion, which survived in that region during the 

 glacial period, or whether it was a post-glacial immigration. 



I. 



In considering the present distribution of the Naiad fauna of North 

 America it is to be borne in mind that while our knowledge as yet 

 is only fragmentary, and there is a great deal more to be learned 

 before definite final conclusions can be drawn, nevertheless there are 

 certain fundamental facts which seem to be reasonably well estab- 

 lished, and with which such tentative deductions as we may attempt 

 to make at the present time must be in agreement. 



In the first place, it seems to be well established that the peculiar 

 North American Naiad fauna originated west of the Mississippi, in 

 the region extending from Utah and Colorado north to Athabasca 

 and Saskatchewan, in British America. 



The earliest forms of recognizable Naiades that are known are 

 from the Triassic and a few more are known from the Jurassic. 

 All these forms are simple and comparatively uniform in their char- 

 acter. But towards the end of the Cretaceous Period, there was, 

 for some reason or other, an extraordinary epidemic, as it were, of 

 mutation in this group, and, in the rocks that were laid down in 

 these western lands at that time, are to be found prototypes of many 

 of the modern groups, which are to-day characteristic of the recent 

 fauna. 



In the second place, it is to be kept in mind that north of the line 

 of glaciation, the entire system of drainage was radically changed as 

 one of the results of the Glacial Period. 



Thirdly, assuming the general proposition that the center of dis- 

 tribution of a group must be considered the region of the greatest 

 abundance of individuals and the greatest diversity of specific de- 

 velopment, it would seem to be reasonably well established that the 

 present fauna of the Mississippian region has spread out from two 

 great centers ; the one on the east, in the head waters of what we 



