Remarks on the Unios of the United States. 43 



names. — Lamark has made three or four species in this 

 manner of the U. fluviatilis. 



There is also a striking resemblance between some of the 

 Unios of the Atlantic States and some species found in our 

 Western waters; thus the U. nassutus of the Delaware, 

 approximates so nearly the TJ. rectus Lam., often called 

 U. prcelongus, of the Ohio, that on comparing a number, 

 we have been almost led to the conclusion that they are 

 merely varieties of the same species, produced by difference 

 of food and locality — they are certainly as much like each 

 other as the U. fluviatilis of the two rivers. That one 

 sh ••"Id be a much thicker and larger shell than the other 

 is no objection; for we find that those specimens of the 

 Unio taken from deep water, are generally thin and fra- 

 gile, while individuals of the same species found in shallow 

 and sluggish streams, are much more ponderous — Again, 

 the U. carinifera of Lamark, which is a true Fluviatilis, 

 very much resembles some specimens of the U. alatus in 

 its wing like appendage, though there can be no doubt that 

 this last shell belongs to a distinct species, if not to another 

 Genus. If we were authorized to adopt the theory that 

 some of the described species of Unio generated promis- 

 cuously together, most of our present difficulties might be 

 very readily removed. Certain varieties in the contour of 

 many species we know take place in consequence of a pre- 

 ternatural enlargement of the shell in certain directions, 

 owing probably to a disease of the animal, which has pro- 

 duced an elongation of his mantle in that direction. The 

 amount of this monstrous growth can be readily determined 

 by the configuration of the impression made by the mantli- 

 of the animal on the surface of the nacre, near the margin 

 of the shi II. Besides the above regular but preternatural 

 formation, many singular distortions of the Unio frequent- 

 ly occur. Not to mention others, in my cabinet, there ' 



