260 SIX NEW SPECIES 



The genus Unio, established by Bruguieres, and placed by 

 Cuvier* in his fourth class of moUusques, les acephales, and 

 second family, acephales testaces, or the mytilaces ; and by 

 Lamarck in his eleventh class, conchiferes, first order, conchi- 

 feres dimyaires. thirteenth family, les nayades; is to the con- 

 chologist one of the most interesting of all the genera. Re- 

 cent American writers on the subject have added many new^ 

 species to this genus, and other new ones are almost daily dis- 

 covered. 



I propose now to add six species, which I believe to be dis- 

 tinct from any hitherto described. In doing which, I give 

 very exact descriptions accompanied by drawings, with a 

 hope they may not hereafter be confounded wath other species. 

 . The constant and perplexing changes which the species of 

 this genus assume have led even the accurate Lamarck into 

 the error of describing several varieties as diiferent species ; 

 and it is not without due hesitation and caution that I am in- 

 duced to add the present. It has been doubtful with some 

 conchologists whether the species of the genus Unio are not 

 the mere varieties of one speciesf. To the naturalist, who has 

 had the opportunity of examining numerous specimens, the 

 gradations are so interesting, and at the same time so perplex- 

 ing, that he is lost in the maze of their changes, and he seeks 

 almost in vain to draw a distinctive line between them ; for 

 even the tuberculated shells sometimes pass by almost insensible 

 gradations into smooth ones. Although this line may not 

 always be satisfactorily draMTi, I think their division into spe- 

 cies should be retained, if it were only for the sake of system. 



The comparative anatomist finds in the animal of the Unio 

 an organization very far advanced towards a state of perfection. 

 Lamarck places it, in his scale of perfection, higher than in- 

 sects, and we cannot be surprised at this, when we examine 

 its structure and find it possessed of brain, heart, branchiae, 



* Regne Animal, vol. ii. p. 453. 



t The following genera, separated from the genus Unio, dipsas (Leach), hy- 

 ria (Lamarck), alasmadonta (Say), damaris (Leach), cannot in the opinion of 

 Mr Swainson retain their station among the genera. 



