AMMONITES. — ZOOLOGICAL, ETC. 



325 



and Angulicostati) are only found in the two lower divisions, 

 the Neocomien and gault. Two groups, {the Tuberculati and 

 Rhotomagenses) are found, on the contrary, in the two upper 

 divisions, the gault and chloritic chalk ; the first most common 

 in the gault, the other in the chloritic chalk. After these dis- 

 tinctions, there remains one group, the Fleoouosi, which is en- 

 tirely confined to the lower Neocomien. We thus see that 

 although some groups are more or less largely diffused over 

 two or three divisions, they nevertheless appear more par- 

 ticularly to characterise some one or other. 



Of the four groups which contain species, common both 

 to the Jurassic and Cretaceous formations, one, the Hetero- 

 phylli, is composed of species which vary according to the 

 formation in which they are found, and will always serve to 

 distinguish them. Two others, the Macrocephali and Planu- 

 lati, are peculiar to the Jurassic formation; and if they 

 contain some species belonging to the Cretaceous formation, 

 they are uncertain, as I have not been able to discover the 

 lobes, and they are all found in the lowest division in con- 

 tact with the Jurassic formation; being the last traces of 

 these forms, indicating that nature has proceeded, in this 

 instance, more gradually than in the other groups. There 

 now only remains the group of the Fimbriati, which, as we 

 see by the species, is most common in the Neocomien. It 

 may be observed, in conclusion, that in all the groups which 

 comprise species, belonging to both formations, they inva- 

 riably are found in the lower divisions of the Cretaceous for- 

 mation, and never in the upper. 



From what has preceded, and which is a faithful expo- 

 sition of the facts I have myself collected regarding Ammo- 

 nites, I think we may draw the following conclusions, which 

 are of high interest as to the order of succession of animals on 

 the surface of the globe, and in the application of their forms 

 to enable us to recognise Formations. 



z 2 



