( 25 ) 



DIVISION XVII. OSTRACE7E. 



GENERA. 



4. Gryph^sA. Lam. M. 



Inequivalved ; inferior 

 valve large, concave, ter- 

 minated by a projecting 

 beak, curved in a spire ; 

 upper valve small, smooth , 

 like an operculum. Fig. 86. 



Hinge without teeth ; One muscular impres- 

 a cardinal hollow, ob- sion on each valve. (1) 

 long, arched. 



of the 



2. Ostrea. Lin. M. 



Shell adherent, inequi- Hinge without teeth, Ligament partly inte- 



valved, irregular, beaks hollow of the inferior rior, inserted in the 



wide apart, becoming very valve increasing wilh cardinal hollow 



unequal with age, the up- age. valves, 

 per valve displacing itself 

 during the life of the ani- 

 mal. Fig. 85. 



«. Edges of the valves simple or undulated, but not folded, 



jfi distinctly folded. (2) 



(1) With the exception of one species (G. angulata) all those known are fossil, 

 and are found in Alluvia, Crag Marl, Portland Hock, Kellovoay Stone, Under 

 Oolite, Blue Lias, Derbyshire Peak Limestone, and at Mans, Brcuille, Nevers, 

 Champagne, Gazonfier, La Rochellc, Roche fort . 



(2) The most remarkable circumstance attending the Oyster, (whose functions 

 seem to he very limited), and which we only observe in it and the Spondylus, is, 

 that in proportion as it grows older and larger, it displaces itself in its shell, and 

 gradually leaves the base of its lower or convex valve, displacing the upper valve 

 at the same time, as well as its ligament ; consequently, the lower valve fre- 

 quently forms a beak or heel of considerable length. The hollow in which the 

 ligament has been successively placed, elongates as the shell increases and becomes 

 a gutter, transversely striated, whilst the ligamental hollow of the upper valve 

 scarcely grows at all. Its fecundity is astonishing. The Osirca cdulis is that 

 which is most frequently eaten. Foss. in the London Clay, Bedford Limestone, 

 Chalk, Green Sand, Blue Marl, Clay, Coral Rag, Clay in Upper Oolite, Oak-tree 

 Clay, at Mans, Grignon, Angers, Cany, Champagne, Neuville, Maastricht, Monl- 

 Marius, Bourdeaux, Boutonnet, Sceaux, Montmartre, Roqucncourt, Longjumeau, 

 Piedmont, Houdan, Mcudon, St, Saturnin, Vahgnc. 



