159 
whorl overlaps a preceding whorl; for as all polythalamous 
shells are cones more or less rolled up upon the same plane, the 
amount of involution must depend upon the angle at which 
the shell bends round when enveloping the embryo, so that the 
bending angle is really a part of the life history of the animal, 
seeing that each true species appears to observe its own angle of 
involution. The fifth feature is the presence or absence of the 
Aptychus and its form and structure, when present, in the 
different genera. A very remarkable specimen of a large Apty- 
chus from the Inferior Oolite of Leckhampton hill, belonging 
perhaps either to A. Sowerby: or A. Parkinsoni, was exhibited. 
It was upon these characters that Waacen constructed his 
arrangement of the genera in the scheme he proposed in the 
memoirs referred to. As additional genera have been erected 
by subsequent authors, derived from certain groups of species 
originally put together by WaaceEn. 
Another step in their classification was proposed by Professor 
Neumayer, of Vienna, in his memoir Die Ammoniten der Kreide 
und die Systematik der Ammomitiden. The Ammonites were here 
grouped in four families. 
I. ARcEstips. 
. Arcestes, Suess. | 5. Pinnacoceras, Mojs. 
. Didymites, Mojsisovics. 6. Sageceras, Mojs. 
. Lobites, Mojs.. | 7. Amaltheus, Montfort. 
. Ptychites, Mojs: 8. Schloenbachia, Newmayr: 
m wh 
II. Troprpipz. 
12. Rhabdoceras, Suess. 
13. Cochloceras, Hauer. 
9. Tropites, Mojs. 
10. Trachyceras, Laube. 
11. Choristoceras, Mojs. 
III. Lyroceratiwa. 
17. Baculites, Zam. 
18. Phylloceras, Suess. 
14. Lytoceras, Suess. 
15. Hamites, Parkinson. 
16. Turrilites, Lamarck. 
