65 



O S T R E A. 



A. Valves furnished with ears, and radiate ; 

 Pecten, 



1. Equilateral 5 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. 



(%. 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 be- 

 tween them than is usually admitted. The ani- 



F 



