Remarks on the Unios of the United States. 41 
But this is not all. — Dr. H. has committed another error 
in this unfortunate note. The animal to which he refers us 
as a variety of the S. variotata, has no relation to that spe- 
cies whatever — it is a variety of the S. glutinosa, and is the 
very animal which I noticed as being found in this state by 
a friend, who informed me that the spots, when the animal 
was alive, were of a silvery hue. (See Contributions, p. 6.) 
The colour of this animal, like that of many others of this 
Genus, is entirely changed by the alcohol in which it is 
preserved — and hence the absolute necessity of describing 
only from living specimens. No one is more fully aware 
of the perplexities which embarrass this interesting subject 
than myself — and none who more justly appreciates the 
merit, the labour, and the assiduity of Dr. Harlan. 
Some remarks on the Unios of the United States, with a 
description of a new species. By Jacob Green, A. M. 
Prof, of Chem. in Jeff. Med. Col. Read April 23, 1827. 
There is perhaps no genus in Conchology more perplex- 
ed and uncertain in its species than that of the Unio. The 
shells which belong to this genus are exceedingly abundant 
in all our fresh water streams; and many of the species as- 
sume such a variation in shape, colour, and marking, and 
even in the position and structure of the teeth, that it is ex- 
ceedingly difficult to decide upon those characters which 
should determine a species and those which mark merely 
varieties. If we take for example the Unio fluviatilis,'^ 
* I have no doubt that the species known commonly by the name of 
Unio purpureus, is the same with the Mytilus flumatilis fig-iired by Lis- 
ter, t. 157, fig-. 12, and described by Dillwyn, from Gmelin, under that 
name. — I have, therefore, been obliged by the rules of Nomenclature, 
now so strictly observed in Natural History, to restore the original spe- 
cific name of Gmelin to this interesting- shell, so well known by the 
name of Purpurms. 
