Mr. Barnes on the Genera Unio and Alasmodonta. 117 
If. Coror of Naker. 
Ill. Surrace. 
The eight margins explained. Every Bivalve shell ma 
be supposed to be circumscribed by an octagon, which will 
e more or less irregular, according to the shape of the 
shell. The eight sides of the octagon will represent the 
eight margins, as will be seen by the following figure. 
Dorsal or hinge 
* 
Posterior 
Basal 
This distribution of the circumference of the shell, tends 
very much to precision in the language of description, for 
if it be said that any particular margin is rounded, arcuated 
or emarginate, the part intended cannot be mistaken. To 
go into an explanation in general, of terms used in wage 2 
tion, would carry us too far from our present purpose. We 
tefer to Burrow. 
We come now to the description of species of the Unio, 
which we propose to distribute into five* sections, by the 
form of the Cardinal teeth. 
UNIO. 
Generick character from VV. Lamarck. 
Shell transverse,' equivalve, inequilateral, free, beaks de- 
corticated,? somewhat carious, (presque rongés) Posterior 
* M. Lamarck makes two sections, the principal distinction of which is, 7n 
en créte and en créte, applied to the Cardinal tooth.: 
