Gasteropods. 27 



announced in other publications, I have demonstrated, in re- 

 gard to a pretty considerable number of species, that these 

 identifications are merely exaggerated approximations of 

 species often very much a.like, but, notwithstanding, specifi- 

 cally distinct. 



The Gasteropods do not seem at first sight capable of af- 

 fording much interest in the point of view in which we are 

 now considering the different classes ; in fact, the Gastero- 

 pods of the palaeozoic formations, and even those of the se- 

 condary formations, with the exception of a portion of those 

 belonging to the chalk, have not yet been sufficiently studied 

 to admit of being compared, with an entire knowledge of 

 causes, with the living species. I shall, therefore, merely 

 remark, that the two types of shells, which we distinguish in 

 the living state, that with the opening entire, and without 

 canal or notch for the respiratory tube, is the most ancient, 

 and is alone met with in the palaeozoic and in the ancient se- 

 condary formations ; while that which has a siphon, does not 

 make its appearance along with the former till after the lias, 

 when it assumes a preponderance, always becoming more 

 marked, in the tertiary formations and in the actual creation. 

 It is a rather singular connection that these ancient Gaster- 

 opods have a greater resemblance in certain respects to our 

 terrestrial and fluviatile shells than to marine species ; wit- 

 ness those numerous species belonging to the Jurassic and 

 triassic formations, which have been referred without suffi- 

 cient cause to Melania or the neighbouring genera. We 

 perceive in this fact something analogous to what I pointed 

 out many years ago with regard to the fossil fishes of the 

 secondary formations, which, although belonging to extinct 

 genera, have a greater resemblance to certain fresh-water 

 fishes of the present day than to any marine fish. 



The numerous special works which have been published on 

 the Cephalopods, living and fossil, from the monographs of 

 MM. de Ferussac and D'Orbigny, down to the most recent 

 productions of MM. de Buch, Miinster, Voltz, Owen, D'Or- 

 bigny, Valenciennes, and others, have made this class well 

 known, and it is one of the most carefully studied of the ani- 

 mal kingdom. It is not, therefore, difficult to seize the na- 



