302 A NATURALIST IN THE GREAT LAKES REGION 



There are more species of fresh-water clams in this south- 

 eastern area, with Chattanooga as its approximate center, than 

 in all the rest of the world. The number of species decrease as 

 you go farther and farther away from this center. Thus, while 

 Maine has only 10 species of Unionidae, Michigan, much nearer 



FIG. 451. Distribution of certain typical deciduous trees 



along the migration route, has 61, Illinois 88, and Alabama 256. 

 Of the Pleuroceridae Maine has none, Michigan 10, Illinois 28, 

 while Alabama has 302. Bryant Walker (Distribution of the 

 Unionidae in Michigan,, Michigan Academy of Science, 1898) 

 considers that the Michigan Unionidae are almost entirely 

 Mississippian, only two species, Anadonta fragilis and Unio 

 complanatus having come from the east. These clams must 



