Mr. Lea on the Naiades. 541 



in the Unios, is Say, in 1818, who described a few species in the 

 American edition of Nicholson's Encyclopedia. Rafinesque, who 

 in 1820 described more than sixty species; Barnes, who followed 

 in 1823, and as to Mr. Isaac Lea, who has had every opportuni- 

 ty of profiting by the labours of his predecessors, and of doing 

 justice to them, he first dates in 1827. Nevertheless, in Silli- 

 man's Journal, it is now stated, that Lea " is pre-eminenl among 

 all the labours of this kind," whilst all notice of Rafinesque and 

 and his sixty species is entirely excluded. 



At page 170, it is said, " the number of species put forth by 

 Mr. Lea, as new, is so great, as at first to excite the suspicion, 

 that many of them must eventually prove mere varieties of one 

 another." At page 10, of Mr. Poulson's translation, we have 

 the following passage : — " A number of species, so great as to 

 quadruple this genus, presenting infinite anomalies in form and 

 structure, is a very remarkable fact, and occasioned doubts re- 

 specting the annunciation of its characters." Considering that 

 Mr. Lea's eulogist had never heard of Mr. Rafinesque's mono- 

 graph, there is a surprising accordance in the passages, which 

 are founded in truth. It is the general opinion of conchologists 

 that a very large proportion of these species are mere varieties, 

 and that any list of species brought forward at this early period 

 of the study of our Unios, will bear a reduction of at least one 

 half. 



Mr. Lea, himself, page 2 of his paper, read before the Ameri- 

 can Philosophical Society, Nov. 2, 1827, says — "The constant 

 and perplexing changes which the species of this genus assumes, 

 have led even the accurate Lamarck into the error of de- 

 scribing several varieties as dilTerent species, and it is not 

 without due hesitation and caution that I am induced to add the 

 present (six species.) It has been doubtful with some concholo- 

 gists, whether the species of the genus Unio are not the mere va- 

 rieties of one species." 



" The gradations are so interesting, and at the same time so 

 perplexing, that he (the naturalist) 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 drawn, I think their divi- 



