140 



thp: geologist. 



of which ranges on the mesial line of the back, which renders 

 it more or less angular. Septa formed of lobes and saddle 

 divided into equal parts. The dorsal lobe is longer than the 

 superior lateral lobe. This group differs from the Armati by 

 its back, which is provided with several ranges of tubercles, 

 one of which is median, by its equal lobes and by its dorsal 

 lobe, which is always the longest. All the species belong to 

 the middle Cretaceous rocks {A. Rhotomagensis, TToolgaru 

 Carolimis, Verneuilianus, Pailleteanus, Fleuriansianus, Man- 

 ielli,^ Papalis, vertebralis, Deveriamis, rusticus, Renauxianus, 

 from the Chloritic chalk, or Upper green sand, and A, Lyel- 

 H,f from the Gault). 



SPECIES HAVING THE BACK EXCAVATED, PROVIDED WITH 

 TUBERCULES ON THE SIDES. 



9th Group. The Dentati, Buch,— Shell more or less 

 tumid, ornamented with ribs, often bifurcating around the 

 umbilicus, where they generally form a tubercle. The ex- 

 tremities of the ribs rise in a salient manner on each side of 

 the back, which is excavated in the middle. Septa formed 

 of lobes divided into unequal parts, and of saddles, generally 

 formed of equal parts. Dorsal lobe, equal or shorter than the 

 superior lateral lobe. All the species belong to the lower 

 Cretaceous rocks. First division ; species having the tuber- 

 cules unequal on the sides of the back {A. vermcosus, from the 

 Neocomien ; A. Interruptus {dentatus, Sow.) denarius, splen- 



* The A. Mantelli, without having the dorsal tubercle, is provided with 

 the same disposition of lobes, and ranges of lateral tubercles. 



t A. Lyelli is the only species of the Gault, and, at the same time, the 

 only Ammonite of this group which, having all the tubercles and equal 

 lobes of the other species, has, nevertheless, the dorsal lobe shorter. It 

 is one of those beautiful exceptions which prove the difference of forms 

 according to the Formation. Looking at its unsymmetrical Septa, we 

 ought, perhaps, to form a separate group for it. 



