65 
O ST REA. 
A. Valves furnished with ears, and radiate; 
Pecten. 
1. Equilateral; ears equal. (Plate IX. Fig. 1.) 
2. Ears unequal. (Fig. 2.) 
- 3. Valves more gibbous on one side ; hinge 
rather oblique ; ears short. (Fig. 3.) 
B. Rough, and generally plated on the outside. 
(Fig. 4.) 
C. Hinge with a perpendicular grooved line. 
(Fig. 5.) 
Shell generally inequivalve, more or less eared. 
Hinge without teeth, having a hollow cavity or 
sinus, and in many instances lateral transverse 
grooves or furrows. 
Shape of the pectinated division orbicular, at- 
tenuated towards the hinge, eared ; of the other 
divisions orbicular, lobed, or produced towards the 
apex. Much as the divisions A. and B. vary from 
each other, there seems more generic affinity 
between them than is usually admitted. The 
animals may not be specifically alike ; but the 
F 
