322 



THE GEOLOGIST. 



A. Regularis. A. Tuber culatus, 



Splendens, Verrucosus, 

 Tardefurcatus. 



Although this character is not confined to the gault, as 

 we find it in many species of the Neocomien and in some^ of 

 the chloritic chalk, I think it may be pointed out as the 

 most general amongst the Ammonites of the gault. 



3rd. A character which is more rare in the other forma- 

 tions is observable in the Ammonites of the chloritic chalk 

 or upper green sand ; that of ha\dng several lateral ranges, 

 and frequently a median range of tubercules, on the sides or 

 back. We find it in the following fifteen species of Am- 

 monites out of twenty-seven^ being rather more than one half. 

 A. Bravaisianus. A. Renauxianus. 



Carolinus. Rhotomagensis, 



Deverianus. Busticus, 



Fleuriansianus. Varians. 



Falcatus, Verneuilianus. 



Lafresnayeanus. VeiHebralis, 



Mantellii, Woolgari. 



Papalis. 



We may then consider these ranges of tubercules as cha- 

 racteristic of the greater portion of the Ammonites of the 

 chloritic chalk. 



In conclusion, the dominant characters in each of the groups 

 of the Cretaceous formation, when confined to purely exterior 

 forms, are in the Neocomien the marks of periodical ces- 

 sations of growth; in the gault, two ranges of tubercules 

 near the back ; in the chloritic chalk, more than two ranges 

 of tubercules near the back. 



When applying more certain and marked characters, arising 

 from the union of external and internal forms, we find the 

 groups of Ammonites, which I have specified when treating 

 of their zoological characters, divided into different forma- 

 tions, as under : 



