AMMONITES — ZOOLOGIGAL, ETC. 



39 



himself in describing the lobes and saddles of the septa of 

 ammonites, and applied them to the formation of groups, 

 divided according to these characters. Nevertheless, owing 

 to the perishable nature of these fossils, and the difficulty of 

 studying them collectively and with general views. Natura- 

 lists have been prevented occupying themselves usefully in 

 their study. The pubhcation of the Paleontologie Fran- 

 9aise^^ having placed an immense stock of materials at my 

 disposal, I have been engaged, for a whole year, in a minute 

 examination of ammonites, in a zoological and geological 

 point of view. The comparative study of a hundred and 

 forty-four species of ammonites from the cretaceous rocks, 

 and of at least an equal number from the Jurassic formation, 

 and of thousands of individuals of all ages, have enabled me 

 to discover many facts which appear to me interesting by 

 their appUcation to Geology, and on account of the zoological 

 characters of species, where examined with reference to the 

 external and internal modifications arising from age, sex, or 

 change in indi^-iduals. I intend to give the results of this 

 extended research, in the hope of enlightening science 

 relative to beings which have totally disappeared and which 

 are only found enclosed in the beds of the eartVs crust. 



ZOOLOGICAL CONSIDERATIONS. 



§ 1. THE EXTERNAL CHARACTER OF AMMONITES. 



Composition of the shell. — The shell amongst the ammo- 

 nites, and 1 may also remark in all the family of Ammonidcs, 

 is very different in its texture from that of the Nautilus and 

 Spirula ; for example, in the latter genera, it is composed of 

 two very distinct portions, the one internal, always nacreous, 

 and brilliant, of which the septse are always formed ; the 

 other external, thin, opaque, ornamented, (amongst the 

 Nautili) by coloured zones. From collecting a great number 



