338 ANIMAL OCCUPIED THE LAST CHAMBER. 
Mr. De la Beche has shcMTi that the mineral 
condition of the outer chamber of many Ammo- 
nites, from the Lias at Lyme Regis, proves that 
the entire body was contained within it ; and 
that these animals were suddenly destroyed and 
buried in the earthy sediment of which the lias 
is composed, before their bodies had either un- 
dergone decay, or been devoured by the crusta- 
ceous Carnivora with which the bottom of the 
sea then abounded.* 
As all these shells served the double office of 
affording protection, and acting as floats, it was 
necessary that they should be thin, or they 
M'ould have been too heavy to rise to the surface : 
it was not less necessary that they should be 
strong, to resist pressure at the bottom of the 
sea; and accordingly we find them fitted for this 
double function, by the disposition of their ma- 
Armatus, A. Sowerbii,) affords a strong argument against the 
theory of their fiaving been internal shells. These spines which 
have an obvious use for protection, if placed externally, would 
seem to have been useless, and perhaps noxious in an internal 
position, and are without example in any internal structure with 
which we are acquainted. 
* In the Ammonites in question, the outer extremity of the 
first great chamber in which the body of the animal was con- 
tained, is filled with stone only to a small depth, (see PI. 36, 
from a. to b.) ; the remainder of this chamber from b. to c., is 
occupied by brown calcareous spar, which has been ascertained 
by Dr. Prout to owe its colour to the presence of animal matter, 
whilst the internal air chambers and siphuncle are filled with 
pure white spar. The extent of the brown calcareous spar, there- 
fore, in the outer chamber, represents the space which was 
