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. variolata, 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 MytWas Jluviati/is figured by Lis- 

 ter, t. 157, fig. 12, and described by Dillwyn, from Gmelin, under that 

 name. — 1 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 Purpureus. 



