40 



THE GEOLOGIST. 



of fragments of Nautili, Ammonites, and Hamites, from the 

 beds of clay and gault of the Boulonnais, where these fossils 

 are admirably well preserved, and studying them on the spot, 

 I observed that fossilization had not changed the two distinct 

 coats of the shell of the Nautili in the least, whilst the ammo- 

 nites always presented the nacre uncovered, without the 

 opaque external pellicle. As I have observed in A. denarius^ 

 lautus, auritus, splendens, cristatus, Bouchardianus^ tubercu- 

 latus, etc. and have found the nacre is always uncovered in 

 Hamites attenuatus, and it shews externally the lines of 

 growth ; so, besides the distinguishing character already 

 observed, the Ammonites have a very distinct structure in 

 their shell, which appears to be common to all the genera 

 having a dorsal siphon, and sinuous or foliated septse (cloi- 

 sons decoupees) composing the family of the Ammonidse. 



Thickness of the Shell, — In general the shell of the Am- 

 monites is thin; but there are many exceptions to this rule, 

 and I possess some, the thickness of which, especially on 

 the sides, is not less than five millimetres. The thickness is 

 exceedingly variable according to the species and the parti- 

 cular part of the specimen. Tf there are ribs, transverse 

 points of junction, tubercles, spines, or no matter what as- 

 perity or salient portion, the shell is always thickest at that 

 part and often double the thickness of the rest. I think I 

 can explain this fact by the necessity the animal would feel 

 of levelling, by an internal addition, the internal surface of 

 the shell, so as to render it more commodious, in the same 

 way as we observe amongst the gasteropodous and acepha- 

 lous mollusca. 



Differences between the internal mould and the external ap~ 

 pendages of the shell, determined by the irregularitij in thick- 

 ness of the shell. — What I am going to observe as to the un- 

 equal thickness of the shell according to the parts, amongst 

 the Ammonites, is not of limited interest, but is applicable to 



