M. Akide d'Orbigny on Living and Fossil Molluscs. 69 



and altogether distinct according to the classes to which they 

 belong, 1 must mention, in a few words, some of these gene- 

 ral characters. 



" Shells are by no means deposited in the earth's strata, 

 as certain individuals have supposed, according to their speci- 

 fic weight ; they are found absolutely in the same conditions, 

 according to which they are at present deposited in the sea 

 on the shores, or, as we find them in modern deposits recently 

 left by the sea.* Bivalve shells, for example, are in their 

 normal position, that is to say, placed vertically, the side of 

 the tubes upwards, the mouth downwards, in the argillaceous 

 or calcareous beds of an infinite number of places belonging 

 to all the different epoch s-t They have been carried along 

 by the currents, and deposited under the waters in horizontal 

 banks,^: or else heaped up on the shores by the waves.| In 

 the first mentioned case, the bivalves are in their place, as I 

 have mentioned ; the gasteropods have the mouth down- 

 wards. In the second case, the shells are deposited by 

 chance, according to their forms ; the flattest will be hori- 

 zontally on the side, as the ammonites and the bivalves, and, 

 finally, each will be found in the position most favourable to 

 the equilibrium of the whole ; but the gasteropods will be 

 found with the mouth sometimes upwards, at other times 

 downwards. In the third mentioned ease, the shells still 

 preserve in some degree the position relatively to their form, 

 and the equilibrium of the whole ; at the same time, as they 

 are not deposited by a slow action, but by a sudden impul- 

 sion, they are found in all positions, without following any 

 certain rule. It may be easily understood how we may de- 



* Those of the bay of Aiguillon, in the confines of the departments of La 

 Vendee and Charente-Inferieure. 



t I have seen them so placed in the inferior lias of Semur (C6te-d'(Jr), in the 

 inferior Oolite of Coulie (Sarthe), in the Kimmeridgean stage of Havre, in 

 those of Chatebaillon (Ijliarente-Inferieure), and in the Portlandian stage of St 

 .Jean D'Angely, in the same department, &c. &c., in the Turonian stage of 

 Montagnt's des Comes (Ande). 



J At Bayeux and Montiers, in the lower Oolite ; at Luc, in the large Oolite 

 (Calvados), &c. &c. 



§ In the localities of the Coral-rag, which I have already mentioned, at tit 

 Mihicl (Meuse), at Tonncrre ("Yonnej, &c. 



