THE CEPHALOPODA. 
MODEEN CLASSIFICATION OF THE AMMONITID^. 
The essential characters on which Von Buch and d'Orbigny based their Ammonite 
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 ought 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 AMMONiTiDJi; 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 : 
groups were derived — 1st, from the greater or lesser complication in the ramifications of 
HamuUna, d'Orb. 
Helicoceras, d'Orb. 
Heteroceras, d'Orb. 
Pfyclioceras, d'Orb. 
Bhabdoceras, Hau. 
Scaphites, Park. 
Toxoceras, d'Orb. 
Turrilites, Lam. 
Anisoceras, d'Orb. 
Buculina, d'Orb. 
Buculites, Lam. 
Choristoceras, Hau. 
CocJdoceras, Hau. 
Crioceras, Lev. 
Hamites, Park. 
