488 FOSSIL INVERTEBRATED ANIMALS. 



of detaching the posterior part of their muscle, without having; 

 need of this plate, for it is not found in their shell. 



The species in this genus of oysters are so numerous, and 

 their form so varied, that it is very difficult to make exact 

 divisions of them, for the intermediate species blend, as it 

 were^ these divisions one into another. M. de France, for the 

 convenience of studying them, divides them — 1st, into those 

 whose valves are simple or waved — 2nd, those in which the 

 lower valve is folded and toothed on its edges, and the upper 

 valve plane, with the edges smooth — 3rd, those which are co- 

 vered with folds, and have both edges of the valves toothed. 

 The shells of this last division are found in the strata of the 

 chalk, and those which are anterior to that formation, but 

 never in those which are more recent. 



Gryphites is the name under which are designated the 

 different species of fossil GryphcBCB. This bivalve shell is re- 

 markable for the inequality and dissimilitude of its two por- 

 tions. The lower valve is very concave, and terminated by a 

 recurved crook. The upper valve, on the contrary, is flat, and 

 seems destined to cover the other and serve it as a kind of 

 opercle. Each of these has but a single muscular impression, 

 and the hinge has but one cardinal fosset, which is arched and 

 without teeth. 



The gryphites are found in the argillaceous limestone which 

 borders the red and variegated sandstone, and which some- 

 times immediately covers the primordial stratum. This par- 

 ticular limestone, which often receives its denomination from 

 the gryphites, is very frequently seen on the confines of the 

 coal strata, and if it does not precisely belong to the same 

 epoch, it is probably but little subsequent to it. It precedes 

 the red sandstone from Metz to Sarrebruck ; — it is found in 

 Burgundy, in the environs of Couches, just before we come to 

 the coal strata of Creusat. It shews itself in Normandy, in 

 the village of Pont-Rond, near the pit-coal of Litry, and in 



