FOSSIL SHELLS ILLUSTKATED BY RECENT. 31;} 
The prodigious number, variety, and beauty, 
of extinct Chambered shells, which prevail 
throughout the Transition and Secondary strata, 
render it imperative tliat we should seek for 
evidence in living nature, of the character and 
habits of the creatures by which they were 
formed, and of the office they held in the ancient 
economy of the animal world. Such evidence 
We may expect to find in those inhabitants of the 
present sea, whose shells most nearly resemble 
the extinct fossils under consideration, namely, 
in the existing Nautilus Pompilius, (See PI. 31, 
Fig. 1), and Spirula, (PI. 44, Figs. 1, 2).* 
ginal simplicity of structure ; a structure which remains funda- 
mentally the same in the Nautilus Pompilius of our existing seas, 
it was in the earliest fossil species that we find in the Transition 
“'Irata. Meantime the cognate family of Ammonites, whose shells 
"'ere more elaborately constructed than those of Nautili, com- 
menced their existence at the same early period with them in the 
fransition strata, and became extinct at the termination of 
the Secondary formations. Other examples of later creations 
of genera and species, followed by their periodical and total 
extinction, before, or at the same time with the cessation of the 
Ammonites, are afforded by those cognate Multilocular shells, 
namely, the Hamite, Turrilite, Scaphite, Baculite, and Belem- 
nite, respecting each of which I shall presently notice a few 
particulars. 
* I omit to mention the more familiar shell of the Argo- 
nauta or Paper Nautilus, because, not being a chambered 
Species, it does not apply so directly to my present subject ; and 
nlso, because doubts still exist whether the Sepia found within 
Ihis shell be really the constructor of it, or a parasitic intruder 
'"to a shell formed by some other animal not yet discovered. Mr. 
t^roderip, Mr. Gray, and Mr. G. Sowerby, are of opinion, that 
'his shell is constructed by an animal allied to Caiinaria. 
