OF THE FAMILY OF NAIADES. 421 



39. U. iris, Lea. 



40. U. zig-zag, Lea. 



41. U. patulus, Lea. 



Conchologists have with great reason complained of the 

 extreme difficulty of identifying Lamarck's species of the ge- 

 nus Unio. Mr Barnes says, "In most cases wherever M. 

 Lamarck can find a difference, though by his own account 

 hiothing remarkable,^ he makes a different species;" and Mr 

 Swainson declares that "one half the species which he has 

 enumerated" cannot be determined on account of the short 

 descriptions and want of figures. The truth of these remarks 

 I have felt severely whenever I have had occasion to consult 

 this author for the genus ; and, with the hope of clearing the 

 path in a measure of those who may follow me, I propose to 

 give here the results of examinations of his species jnade at 

 difterent times with much care. 



U. sinuata. This is the Mya margaritifera of Linnajus 

 and other authors, and to which Barnes's Jllasmoclonta arcu- 

 ata is the analogue. Mr B. was not aware, when he descri- 

 bed it, that it was similar. He has recently, in the reclama- 

 tion of his Uniones, resigned this species of Alasmodonta. If 

 Mr Say's genus be admitted, we must of course call this type 

 of Lamarck's Unio, Masmodonta margaritifera, 



U. elongata. There can scarcely be a doubt but that this 

 is a young shell of the above species. 



U. crassidens. It is evident on examination of our author's 

 description of this species and its varieties, and the crassiis of 

 Say and of Barnes, that all the ponderous varieties of our 

 Uniones were brought into these species, neither of which 

 can possibly stand. (See note, page 417.) 



U. peruviana. This species embraces the plicata of Le 

 Sueur, the crassus and undulatus of Barnes, the giganteus of 

 Dr Mitchill's collection, the rariplicata and crassidens of La^ 

 marck, and the imdulata and dombeyana of Valenciennes. 



