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congeners by Mr. Lea, who thus builds his artificial 
genus Symphynota on the ruins of several well de- 
fined and natural genera. Indeed, it is yet sud judice, 
whether or no ail 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 many 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 has 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 Lza, 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 Symphynota, doubtless, 
because it no longer needs the essential character 
of that genus. In Mr. Poutson’s cabinet is a re- 
markable specimen of Unio plicatus, of Say, which 
approaches the alated shells, and appears to have 
been connate behind the ligament; and I found in the 
Alabama river a specimen of Unio trapezoides, Lea; 
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- 

