Pulmonifera of Maine. 55 



nearly the same number of Unionidae absent — showing not only 

 a more general distribution of the former class, but exhibiting 

 also the continual reduction of species of Unionidae as one goes 

 East from the Ohio and Mississippi valleys. In New Brunswick 

 and Nova Scotia we shall undoubtedly observe a still less number 

 of species in this family. The Melanidas so numerous in the 

 West, have not a single representative in all the lakes and rivers 

 of Maine, and only one species is found in New England. 



