320 



POPULAR CONCHOLOGY. 



brought it by the waves, and giving scarcely a sign of life, 

 except the opening and shutting of its valves. 



It is supposed that above one hundred thousand bushels 

 of oysters ( O. edulis) are sold in London every season. 



Dr. Mantell remarks that there are between thirty and 

 forty species of fossil oysters found in Britain alone. He 

 mentions that in the tertiary strata of the London basin 

 there are oyster beds of vast extent, and in some places 

 they are quarried and used for building purposes ; when- 

 ever the strata round London are perforated to a sufficient 

 depth, this oyster bed is reached. Sowerby speaks of 

 immense masses of O. gregarea being found near Devizes 

 attached to each other. 



Gryph^ea. Lam. — Shell inequi valve, free ; lower 

 valve large, concave, with the umbo 

 prominent, incurved; upper valve 

 small, flat, opercular-like ; hinge 

 toothless, with a curved depressed 

 area ; one muscular impression.* 

 — Fossil. 



They are very remarkable for 

 the curved produced beak of the 

 lower valve. Mantell mentions that there are about 

 thirty British species, one of which, G. i?icurva, is found 

 in great abundance in the lias about Cheltenham. 

 Another small species, G. virgula, is so abundant in the 

 Kimmeridge clay, that it constitutes entire layers. The 

 low cliffs on the west of Boulogne harbour, like those of 

 Weymouth, are composed of this clay, and myriads of the 

 Gryphaa are scattered on the shore. f 



Exogyra. Sow. (Amphidonta Fischer.) — Shell 

 resembling Chama in shape, but Ostrea in structure: 



* Sowerby's Conch. Manual. 



f Medals of Creation. 



