116 Mr. Barnes on the Genera Unio and Alasmodonia. 
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 known, 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.) Externat. (B.) Internat. 
Each of these comprehends three divisions, viz. I, Form 
II, Color, III, Surface. With sub-divisions as follows, viz. 
A. ExtTerna.. 
¥. Form anp SuspstTance 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, etic and posterior. 
9. Eight margins 
a. Hinge, or dorsal. 
b. Basal. 
c. Anterior. 
d. a 
e. f. Anterior, dorsal and basal. 
g. h. Pashivicr, dorsal, and basal. 
Il. Coxor of Epidermis. 
Il]. Surrace. 
B. Inrernau. 
I. Form of 
1. Cardinal teeth. 
2. Lateral teeth. 
3. Muscular impressions, or Cicatrices. 
4. Cavity of the beaks. 
