136 
aquis diilcibus/ 3. Tlie comjplanata answering, in description, 
better to our shell, and being the first figured and described. It 
appears somewhat singular to me, that the observant and able 
zoologist, Mr. Say, had not been struck with the similitude of our 
shell to Lister’s figure and description. There is no species more 
common in all our fresh waters, east of the Alleghany Mountains, 
than this ; and nothing could be more likely than that it should be 
among the first to be taken to Europe by the early voyagers to 
America. In accordance, therefore, with the rules of nomencla- 
ture, I have inserted the name of complanatus to the shell de- 
scribed by Mr. Say under the name of purpureus.” — (Trans. 
Philos. Sc. Yol. 3, N. S.) 
I have not, at present, the means of referring to Dillwyn’s work, 
and I can therefore only speak in relation to Lister’s figures, 1. 150, 
above mentioned, and consequently, to any description founded on 
these figures. On reading the above, I was myself struck with the 
singularity of the fact, that I should have overlooked the similitude, 
if any, between the shell and a figure with which I was equally well 
acquainted. But on a re-examination of the figure, I am in my 
turn surprised that any one should have thought of bringing the 
purpureus into comparison with it. How the figure can appear so 
differently to different conchologists, I know not ; but certain it is 
that Gmelin quoted it, with a saving mark of doubt, for Mytilus 
cygneus ! I I quoted it in the year 1819 for Unio crassus, Barnes 
also quoted it for the U. crassus, and now it is supposed to repre- 
sent the shell in question. 
After a careful comparison of the shell with that figure, I can- 
not, with any moderate concession of character, make it correspond ; 
and, moreover, I cannot think that any one will be more success- 
ful in this respect than I have been. But even if a strong likeness 
really existed, two words of Lister’s description, quoted by Mr. 
Lea himself, puts the matter altogether beyond a doubt. The 
words, admodum crassus’’ engraved on the same plate with the 
figures, admit of no misconstruction, and no approximation of the 
purpureus, (which Mr. Lea says, p. 33, note, is never ponder- 
ous,”) but agree very well with a variety of the unfortunate U. 
crassus ; for which I originally quoted the figures, and which is 
admodum crassus.” For these reasons, I think that cause has 
not yet been shown why the name should be changed ; and, there- 
