406 NEW GENUS AND SOME NEW SPECIES 



Uniones, described by naturalists who have written on our 

 shells. 



The genus Unio presents in the waters of the United 

 States, particularly in the rivers west of the Alleghany moun- 

 tains, a number of species almost extending beyond belief. 

 Nature has scattered them here with the hand of profusion, 

 after having formed them with the most harmonising beauties. 



The number of the species adds greatly to the difficulty of 

 distinguishing them, for they glide into each other so insensi- 

 bly through their varieties, that the most experienced are often 

 at fault and perplexed with the difficulty of placing them pro- 

 perly in the most approved systems*. But, although we may 

 at every step meet with these difficulties, I cannot suppose that 

 most of those described as species do not exist ; the fault has 

 been that mere varieties, in the eagerness of authors to make 

 species, have too often been erected into species, and the great 

 Lamarck has committed this error in as great a degree as 

 almost any other writer. 



It is the opinion of some eminent conchologists that the 

 family of the Naiades possesses but one genus, and that the 

 genera into which it is at present divided are only species, 

 and the species varieties. Were we to adopt this division, we 

 should be in a worse dilemma than before ; for we can scarcely 

 imagine bivalves more different from each other in form than 

 are some of our trans-Alleghany species of Unio. 



How totally different is the rectus of Lamarck from the 

 irroratus? (nobis). The first is four times the width of its 

 length, whilst the latter is longer than broad. The one is 

 broad rayed, in fine specimens ; the other possesses dotted 

 lines universally. The triangularis of Barnes is entirely 

 dissimilar to the nasutus of Say, as is also the circulus, 

 herein described, from the lanceolatus (nobis) ; and the same 

 may be said of peruvianus and pictorum. Two species could 



* Swainson says, " Indeed so much uncertainty hangs on the shells of this ge- 

 nus, that the species can only be fixed by ample descriptions and very correct 

 ("mures." — Zool. Illus. Vol. I. t. 57. 



