THE CEPHALOPODA. 219 



MODEEN CLASSIFICATION OP THE AMMONITID^. 



The essential characters on which Von Buch and d'Orbigny based their Ammonite 

 groups were derived — 1st, from the greater or lesser complication in the ramifications of 

 the suture-line, and the character of the lobes and saddles formed therefrom ; 2nd, the 

 position and structure of the siphuncle ; and, 3rd, the form of the spiral of the shell. 



The first, we have already seen, Von Buch considered of primary importance, and on 

 this alone proposed the families, as he called them, in his classical memoir, of which I have 

 given a resume. Subsequent observations, however, made by many independent students 

 have taught them that this character, although an important one, is not so stable as the 

 great naturalist believed — a fact which we have ascertained by noting the changes from 

 simplicity to complexity in the morphology of Goniatites, Ceratites, and Ammonites. In 

 adult life, however, the form of the suture-line is a valuable character, and ouglit to be 

 accurately drawn from careful tracings made on the specimens themselves over the natural 

 lines, and not given as mere sketches by the artist, as is often the case ; in tracing the 

 lines we have a natural autograph, by the other mode a mere ideal representation ; so 

 that, whenever suture-lines are exposed and can be traced on the specimen, they should 

 be carefully noted and recorded. 



In all the Ammonitid^ the position of the siphuncle is invariably in the centre of the 

 abdomen, and in the outer margin of the greater curvature of the spiral of the shell 

 opposite to the columella or dorsal border. The presence or absence of a keel in which it 

 is sometimes lodged, and the roundness or flatness of the siphonal or abdominal area, 

 all afford specific characters which it is important to note. 



The form of the spiral described by the growth of the shell has been taken by 

 Parkinson, Lamarck, d'Orbigny, Hauer, and Leveille as a basis for the establishment of 

 several genera, and from this character alone we have the following sixteen genera : 



Ancyloceras, d'Orb. 

 Anisoceras, d'Orb. 

 Bucidina, d'Orb. 

 Buculifes, Lam. 

 Choristoceras, Hau. 

 CocJdoceras, Hau. 

 Crioceras, Lev. 

 Hamites, Park. 



Hamulina, d'Orb. 

 Helicoceras, d'Orb. 

 Heteroceras, d'Orb. 

 Pfi/choceras, d'Orb. 

 Bhabdoceras, Hau. 

 SccqjJiites, Park. 

 Toxoceras, d'Orb. 

 Tiirrilites. Lam. 



