116 M.r. Barnes on the Genera Unio and Alasmodonta. 



well as of many Oceanic Bivalves, it might seem most prop- 

 er to commence with that part in describing. But as the 

 generick characters, standing at the head of the genus are 

 supposed to be knovv^n, and are therefore not enumerated in 

 the description, and as the method of commencing with the 

 exterior has been generally adopted, we have not deemed 

 it necessary to depart from the established usage. The 

 parts are two, viz. 



(A.) External. (B.) Internal. 



Each of these comprehends three divisions, viz. I, Form, 

 II, Color, III, Surface. With sub-divisions as follows, viz. 



A. External. 

 I. Form and Substance includes 



1. General outline or circumference. 



2. Substance of the shell. 



3. Disks, right and left. 



4. Sides, anterior and posterior. 



5. Umbones or bosses 



6. Beaks. 



7. Ligament. 



8. Lunules, anterior and posterior. 



9. Eight margins viz. 

 a. Hinge, or dorsal. 

 h. Basal. 



c. Anterior. 



<Z. Posterior. 



e, /. Anterior, dorsal and basal. 



g. h. Posterior, dorsal, and basal. 



II. Color of Epidermis. 



III. Surface. 



B. Internal. 



I. Form of 



1. Cardinal teeth. 



2. Lateral teeth. 



3. Muscular impressions, or Cicatrices, 



4. Cavity of the beaks. 



