the margin or edge opposite to this, is the basal 

 margin ; that in front, the anterior margin ; and 

 that portion of the shell nearest to the observer, 

 the posterior margin. When the shell is held in 

 this manner, the right and left valves are also 

 determined by their relative position to that of 

 the observer. 



At the same time that Mr. RAFINESQUE sub- 

 divides the species into genera and sub-genera, 

 he likewise retains throughout the work, for rea- 

 sons assigned in his preliminary remarks, the 

 genus UNIO as established by BRUGUIERE ; this 

 arrangement will be found satisfactory, because 

 it prevents ambiguity, and will enable the student 

 to adopt his generic terminology at discretion. 



I have much pleasure in stating, that most of 

 the Shells described in the following pages, have 

 been placed in my cabinet by Mr. RAFINESQVE, 

 with his labels and references. 



