AMMONITES— ZOOLOGICAL, ETC. 



137 



DIVISION OF THE AMMONITES INTO GROUPS. 



I shall now occupy myself with the classification of Am- 

 monites into natural groups; and shall develope the ingenious 

 classification of Leopold von Buch, by applying it to the 

 Ammonites of the Cretaceous Period. The following are the 

 results with which the combination of the characters of the 

 lobes and saddles with the external form of the back and 

 other appendages, have furnished me. I am obliged to in- 

 clude the whole of the Ammonites, to enable me to class 

 those of the Cretaceous period. 



SPECIES HAVING AN ENTIRE DORSAL KEEL. 



1st Group. Arietes, Buch. — S/ie/Z ornamented, on the sides, by ribs 

 always simple, radiating, salient. Back square, provided with a mesial 

 keel Syphon salient, placed in the dorsal keel. Mouth elongated into a 

 beak. Septa formed of unequal lobes and saddles.* Dorsal lobe deep 

 as wide, longer than the superior lateral lobe. The lateral saddle mounts 

 rather higher than the rest ; the dorsal saddle is very short. The species 

 of this group are all confined to the lower beds of the lias (A. BucJclandi, 

 obiusus, rotiformis, 8fc). 



2nd Group. Falciferi, Buch. — Shell compressed, furnished laterally 

 with folds, inflected forwards, frequently forming an angle about the 

 centre of their length, without tubercules. Back sharp, salient, keel nar- 

 row, containing the syphon. Mouth complete, provided, on the middle of 

 each side, with salient points. Septa formed of unequal lobes, and 

 saddles almost equal. The dorsal saddle is of immense width, and its 

 auxiliary lobe might be mistaken for the superior lateral one. The latter 

 is always much longer than the dorsal lobe . This group is peculiar to 

 the upper beds of the Lias (A. Serpentinus, Schl. Murchisonce, falcifer, 

 Bifrons, &c). 



3rd Group. Christati, d'Orb. — Shell compressed, ornamented on the 

 sides, with bifurcating ribs, inflected anteriorly, ^vithout forming an 



* These characters of the equal and unequallohes and saddles have been 

 introduced by myself in characterizing groups. As also those of the en- 

 tire mouths. 



