ON THE FAMILY OF RUDISTA. 



135 



other^ therefore^ the respective fauna of the five different 

 zones of Rudista, v/hether in the beds of one epochs or of 

 distinct periods, having been destroyed, and subsequently 

 replaced by others entirely different in their characters, prove 

 that in this series of beings, there was no merging of specific 

 formsj much less of the beds in which they are contained. 



4. — The Rudista, being situated in clearly defined limits, 

 in the cretaceous system, form horizons more or less ex- 

 tended, and which always bear the same relative position to 

 other fossils. 



Thus M. d'Orbigny proves, that the assignment of genera 

 and species to particular formations is not by any means a 

 chance, but that as proved in the instance of the Cephalo- 

 poda also, (Paleontologie Francaise), it is invariable in order, 

 the several fauna being more or less numerous in contents, 

 and is one of the readiest identifications of the chronological 

 history of the Zoology of the globe. 



Ammonites Zoologically and Geologically considered. 



(Continued from p. 111.) 



III. Affinities between the external and internal characters. 



When I search for the affinities which exist between these 

 two series of forms, I can easily prove that there is frequently 

 a perfect concordance, and that the one would serve to guide 

 our judgment as to the others, and to form groups per- 

 fectly natural. In addition to what I have already stated 

 respecting the size of the whorls of the spire when compared 

 with the number of the lobes, I shall offer some examples of 

 species having common internal and external characters. 



I. — Among the Ammonites having the lobes formed of 

 equal parts, 1 have observed a series characterized by having 



