THE NAUTILUS. 19 



an unglaciated area in central North America, on the ground that 

 this peculiar fauna of Lake Erie and the adjoining waters is a relict 

 fauna, the remnant of a pre-glacial immigration from the south, rather 

 than a post-glacial invasion, which has been modified since the dis- 

 appearance of the glacier, the subject becomes one of considerable 

 importance and worthy of careful consideration. 



The study of the geographic distribution of the North American 

 Naiades is one of comparatively recent origin, and it is only within 

 the last fifteen or twenty years that any particular attention has been 

 given to it. The fact is that it is only within that time that suffi- 

 cient data have been accumulated, upon which any reasonable gen- 

 eralizations could be based. The time and efforts of the. earlier 

 generation of students, of which Dr. Lea was the leading exponent,, 

 were wholly taken up in differentiating and describing the new- 

 species as they were collected in various parts of the country. The 

 first attempt to deal in any general way with the distribution of the 

 fauna was that of Simpson, who, in his monumental work, " The 

 Synopsis of the Naiades," published in 1900, not only put the classi- 

 fication of the family for the first time upon a scientific basis, but 

 also separated the fauna into its several main constituents. 



According to Simpson, North America, north of Mexico, is 

 divided into three great faunal areas : on the east and limited on the 

 west by the Appalachian Mountains toward the south, and extending 

 in an indefinite direction towards the north and northwest, is the 

 Atlantic region; on the west coast, bounded by the Rocky Mountains 

 and the Sierra Nevadas on the east, is the Pacific region; while the 

 whole interior portion of the country, extending from the Gulf as 

 far north as Unione life can survive, forms one large province in- 

 habited by the fauna, which he calls the Mississippian. The addi- 

 tional information of the subject that has been accumulated since the 

 publication of Simpson's book has served only to confirm the cor- 

 rectness of his general division into these three great regions, but, 

 as might be naturally expected, certain modifications will have to be 

 made as the results of our increased knowledge of the range of many 

 of the species. So far as the purposes of this paper are con- 

 cerned, it is only necessary to say that, in figure 1, p. 20, I have 

 extended the Atlantic region across the Georgian Bay and about 

 half way along the eastern end of Lake Superior, both on the north 

 and on the south shores. 



