M. Alcide d'Orbigny ow Liviny and FotssH Molluscs. 61 



This subject having been equally the object of my special in- 

 vestigations for many years, both in Europe* and America,-f- 

 I sliall mention some of the results I have obtained up to the 

 present time, until the successive synoptical views, in genera 

 and classes, furnish me vi^ith more complete and definite so- 

 lutions. The following are the conclusions which I can now 

 deduce, — conclusions of great interest for the solution of the 

 important questions respecting the chronological history of 

 animal life on the surface of the earth. 



" Molluscs, considered as a whole, have proceeded accord- 

 ing to the chronological order of the faunas peculiar to the 

 formations, fi'om the simple to the composite. Many genera, 

 it is true, have completely disappeared with the ancient for- 

 mations ;+ others, appearing at a later period, § have like- 

 wise become extinct with the strata of the cretaceous forma- 

 tions ; but the genera, multiplying more and more as we re- 

 cede from the first age of the world, have been replaced, dur- 

 ing the period of the cretaceous and tertiary formations by a 

 multitude of forms which were wanting in the lower beds,|| 

 and these forms are still more diversified in our present seas,!^ 

 vphere they reach the maximum of their numerical develop- 

 ment. 



" No transition being traceable in the specific forms, mol- 

 luscs appear to succeed each other on the surface of the globe, 

 not by a gradual passage, but by the extinction of existing 

 races, and the renovation, or successive creation of species 

 at each geological epoch. 



" Molluscs are distributed in zones, according to the geo- 

 logical epochs. Each of these epochs, in fact, represents on 



* See my Paleontologie Frani-aise, and particularly the Synopsis at the end of 

 each class, vols. i. ii. 



t Paleontologie Je V Amerique Meridionale (Voyage dans V Amerique Meridionale, 

 vol. iii.) See also the Oeologie of the same work. 



J The Orthoceratites, Cirthoceras, Goniatites, Productus, and Spirifeia. 



§ The Ammonites, Toxoceras, Ancycloceras, Ptychoceras, Erioceras, Uam- 

 ites, Acteonella, &c. 



II A multitude of genera have appeared at this epoch ; Voluta, Mitra, Mu- 

 rex, &c. 



^ The number of genera not known in a fossil state is a proof of this; Pe- 

 dum, Magilus, &.C. 



