Mr. Barnes on the Genera XJnio and Masmodonta. 117 



II. Color of Naker. 

 III. Surface. 



The eight margins explained. Every Bivalve shell may 

 be supposed to be circumscribed by an octagon, which will 

 be 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 hins[e 



See Unio J^odosus, 



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 descrip- 

 tion, would carry us too far from our present purpose. We 

 refer to Surrow. 



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 M. Lamarck, 



Shell transverse,' equivalve, inequilateral, free, beaks de- 

 corticated,^ somewhat carious, (presque ronges) Posterior 



* M. Lamarck makes two sections, the principal distinction of which is, non 

 en Crete and en crite, applied to the Cardinal tooth. 



