OF CREATION. 141 



has been supposed, although it is by no means 

 proved, that this tube permits of a change being 

 effected in the specific gravity of the animal, by 

 forcing in a small quantity of fluid in addition to 

 that which the tube generally contains. 



The nautilus is a genus which has been almost 

 universally distributed both in time and space, having 

 been introduced at a very early period, existing ap- 

 parently in all parts of the ancient seas, and continued 

 in some shape or other even up to the present day. 

 The Cephalopoda, as a group, are, however, chiefly 

 characteristic of the first and middle epochs ; and one 

 form was greatly developed during the early portion 

 of the period we are now considering. The genus 

 Ammonites, the one here alluded to, may therefore 

 be properly described when speaking of the lias 

 fauna. There are, however, certain subdivisions of 

 the genus as it now stands, which appear confined 

 to beds of a certain age ; and the annexed figure 

 represents a group peculiar to the older part of 

 the secondary epoch. 



In the true ammonite the Fi 9- 54 



shell is spiral, and coiled on 

 itself in one plane, the whorls 

 at least touching, and not un- 

 frequently enclosing one an- 

 other. It differs from the nau- 

 tilus in several respects, namely, 

 (1) in the substance of the shell 

 itself, which is generally, though 

 not invariably, much thinner AMMONITE OF THE LIAS. 

 than in the latter genus ; (2) in (Group Falci f erL > 

 the form of the aperture and the relative proportion 



