65 



OSTREA. 



A. Valves furnished with ears, and radiate; 



Pec ten. 



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 



