334 Recherches sur les Poissons Fossiles. 



then extend these inquiries to the growth of the vertebrae, 

 to the development of the plates, and the spines of the 

 echinoderms, to the cellules of the polyps, we might hope 

 that a day would not be far distant when we should obtain by 

 this means a glimpse of the duration of the creations, which 

 have successively enlivened the surface of our globe. In 

 presenting the geological results of my researches, I have 

 become acquainted with facts which I have already arrang- 

 ed with regard to the class of fishes. As to the Molluscs, 

 the species whose shell is ornamented with spines, pores, 

 channels, and enlargements regularly disposed on the sur- 

 face, are the most adapted to initiate us into an acquaint- 

 ance with their age, as Cardiums, Chamas, Ranellas, Tritons, 

 the Murex's, Scalaris's : the partitions of the Nautilites, of the 

 Ammonites, and in short, the number of whorls in the outline 

 of the spire of all the species, will yet be sufficient to serve as 

 a term of comparison. The determination of the relative 

 age of analogous species in the different successive geologi- 

 cal formations, will present the most curious results. Science 

 besides would derive from such researches the advantage of 

 reducing considerably the number of species, and above all, 

 of pointing out those differences that depend merely on 

 age. 



All the modifications that we observe in the form and 

 nature of the surface of scales, proceed from the form 

 of the leaves of growth, and the manner in which they are 

 superimposed one upon another. Nevertheless it must not 

 be forgotten, that there are scales (in the Ganoides,) on the 

 external surface of which layers of enamel are deposited 

 in yn-oportion as the leaves of growth are extended, like the 

 enamel of the teeth which covers their crown, while deficient 

 on the bony layers which form their roots. 



The outline of scales varies infinitely in different species 

 of fish, and we even observe the difference of form to be 

 considerable in each individual, according to the place 



