OSTREA 



€e dont aucune autre coquille bivalve n'ofFre d'exemple, 

 si I'on en excepte les Spotidj/les.'''' Upon this we will 

 make only one or two observations, and leave it to be re- 

 ceived or rejected according to tlie judgment of those who 

 have opportunities of studying- more closely the habits of 

 those marine animals. The careful examination of many 

 specimens has not furnished us with any reason for altering 

 our opinion that the Oyster shell is formed, like all other 

 shells, by a deposition of shelly matter from the animal 

 on the inner sides of the valves, consequently that there 

 exists no peculiarity in this respect in the Oyster, inasmuch 

 as if it be necessary for the Oyster to displace itself in its 

 shell, it must also be equally necessary for the animals of 

 all other bivalves to do so too ; and a common observer 

 will soon perceive that the animals of all bivalves recede 



fradually from the base or umbo of their valves. But 

 lamarck further mentions the Spondj/lus as another, and 

 the only other example of the above related displacement 

 of the upper valve, to which we have one fact to object, 

 which we think completely decides the inadmissibility of 

 the conjecture (for we suppose it merely a conjecture); 

 this fact is, that the two valves of the Spondylus cannot 

 be separated without breaking the hinge teeth, conse- 

 quently, unless the animal damages the teeth of its shell, 

 there can be no displacement of the upper valve. 



We believe there exist a considerable number of 

 species of this genus ; but the irregularity of the shells 

 renders the determination of the species extremely difficult. 

 The fossil species are found in almost all the strata down 

 to the Lias ; the species are however so extremely difficult 

 to recognize, that, with one or two exceptions, we dare not 

 venture to indicate positively those which are peculiar 

 to certain strata. 



The Ostrea deUoidea, Sowerby's Mineral Conchology, 

 tab. 148. is characteristic of the Kimmeridge Clay, or 

 Oak-tree Clay of Smith. The O. pulchra, Min. Con. tab. 

 279. appears also to characterize a portion of the Plastic 

 Clay. The O. (Gryphwa) dilatata occurs in the Kello- 

 ways and Cluncli Clay, and the O. (Gri/phcca) incurva 

 in tha Lias. 



FIRST SECTION. 



First Plate, Fig. 1. Ostrea edulis. 



Virginica. 



