20 



cong-eners by Mr. Lea, who thus builds his artificial 

 genus Symjjhynota on the ruins of several well de- 

 fined and natural genera. Indeed, it is yet sub jucUce, 

 whether or no all the Naiades are connate in the 

 young state. When perfect, U. ater, Lea, and U. 

 irapezoides, Lea, are both connate behind the liga- 

 ment, but cabinet specimens do not show it. 



The testaceous connexion of man> species is so 

 extremely thin and fragile, that the action of the 

 current, and the sand shifting on the bars wear it com- 

 pletely away. Unerring nature lias formed it thicker 

 and stronger in the weaker and more fragile species, 

 in order to give support to a portion of the shell which 

 otherwise would be peculiarly liable to injury. In one 

 species of Unio, the ater of Lea, the hinge is connate 

 only whilst the shell is young. As it advances in 

 age, the shell becomes thick and ponderous, and loses 

 its station in the ranks of Syinphynota, doubtless, 

 because it no longer needs the essential character 

 of that genus. In Mr. Poulson's cabinet is a re- 

 markable specimen of Unio j^Ucatus, of Say, which 

 ai)proaches the alated shells, and appears to have 

 been connate behind the ligament; and I found in the 

 Alabama river a specimen of Unio trapezoides, l-ea, 

 which possesses a similar character. The testace- 

 ous matter of the shells of the Naiades, always 

 covers a portion of the ligament, and when not ex- 



