On the Mississippi Valley Unionidce. 



ON THE MISSISSIPPI VALLEY UNIONISE FOUND 



IN THE ST. LAWRENCE AND ATLANTIC 



DRAINAGE AREAS. 



By Chas. T. Simpson. 



The entire Mississippi drainage area is peopled by a pecu- 

 liar Unione Fauna. 1 



The species are exceedingly numerous, and many of them 

 attain great size, or become very solid at maturity. A large 

 number are characterized by strong sculpture in the form of 

 knobes, pustules or plications, or by striking outlines, and 

 the species in general are more richly colored externally or 

 internally than those of any other part of the globe. 



The Atlantic drainage area, including a considerable part 

 of the St. Lawrence River system, is occupied by a very dif- 

 ferent Naiad fauna. As a rule the species are moderate in size 

 and conform nearly to the ordinary oval or oblong-oval Unione 

 type ; they are of light structure, without sculpture or strong 

 angularities and lobes, and are plain colored in nacre and 



The dividing line between these two Unione faunas is not 

 directly on the Height of Land, which separates the St. Law- 

 rence and some of the other Atlantic drainage systems from 

 that of the Mississippi, but it is considerably to the northward 

 and north-eastward of it. 2 



To the westward the Red River of the north, the Sas- 

 katchewan and Mackenzie are largely inhabited by Missis- 

 sippi Valley Uniones, and they are found abundantly in all 

 the great lakes, the southern peninsula of Michigan, the 

 streams in Wisconsin, Indiana and Ohio that drain into these 

 lakes, and well up into Eastern Canada, Lake Champlain and 



s Eelationships and Distribution of the 



naturalist, A A" 177, p, 353. 



a paper by the writer, which will soon be 



