THE NAUTILUS. "23 



while the Lake Erie fauna includes fifteen genera and thirty-nine 

 species. 



Of the Atlantic fauna, three species, Lampsilis nasuta, Anodonta 

 marginata and Alasmidonta marginata varicosa, and perhaps a 

 fourth, 1 are also found in Lake Erie. 



Two species, Margarilana margaritifera, a preglacial immigrant 

 from Europe, and Alasmidonta undulata, do not extend into the Erie 

 basin and have no closely allied representatives there. 



The remainder, though not found in the Lake Erie fauna, are, 

 nevertheless, represented there by closely allied species evidently of 

 a common derivation, as indicated in the foregoing list. Elimina- 

 ting these species, we find the remainder of the Lake Erie fauna to 

 consist of eleven genera and thirty species, which are not repre- . 

 sented in any way in the New England fauna. 



The relation of these two faunas in the region of the Great Lakes 

 region can, perhaps, be best shown graphically by a comparison of 

 the range of two of their characteristic species, which are closely 

 related to each other, and both of wide distribution, viz., Unio com- 

 planatus Dill, and U. gibbosus Bar. 



It will be observed from figure 2, that Unio complanatus ex- 

 tends from the Atlantic region proper, northwesterly across Ontario 

 into Georgian Bay, up the St. Mary's River and along the eastern 

 half of both the north and south shores of Lake Superior, and, so far 

 as we know, probably occupies all of the Canadian region north and 

 east of that line as far as Hudson's Bay and Labrador. On the 

 other hand, Unio gibbosus, the representative of the Mississippian 

 fauna, extends from the Menominee River, the dividing line between 

 Wisconsin and Michigan, entirely around the shore of Lake Michigan 

 and along the south shore of Lake Huron from Mackinac through 

 the St. Clair River, Lake St. Clair, the south shore of Lake Erie, 

 and east as far as the Ottawa River. At that point the two species 

 are found living together in the same stream. It occupies, of course, 

 the entire inland region south of the Great Lakes in Wisconsin, Illi- 

 nois, Michigan, Ohio and western New York. 



1 The specific distinctness of Strophitus edentulus and undulatus is questioned 

 by eminent authority. 



(To be continued.) 



