256 ANIMAL OCCUPIED THE LAST CHAMBER. 



was analogous to that of the inhabitant of tiie Nautilus 

 Pompilius. (See PI. 31, Fig. 1.) 



Mr. De la Beche has shown that the mineral condition of 

 the outer chamber of many Ammonites, 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 com- 

 posed, before their bodies had either undergone decay, or 

 been devoured by the crustaceous 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 would 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 lind them fitted for this double function, by 

 the disposition of their materials, in a manner calculated to 

 combine lightness and buoyancy with strength. 



* In the Ammonites in question, tlie outer extremity of the first great 

 chamber in which the body of the animal was contained, is filled with 

 stone only to a small depth, (sec 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, therefore, in the outer 

 chamber, represents tlie space which was occupied by the body of the animal 

 after it had shrunk within its shell, at the moment of its death, leaving void 

 the outer portion only of its chamber, from a. to b., to receive the muddy 

 sediment in which tlie shell was imbedded. 



I have many specimens from the lias of Whilby, of the Ammonites Com- 

 munis, in which the outer ciiambcr thus filled with spar, occupies nearly 

 the entire last whorl of the shell, its largest extremity only being filled with 

 lias. From specimens of this kind we also learn, that the animal inhabiting 

 the shell of an Ammonite, had no ink-bag ; if such an organ existed, traces 

 of its colour must have been found within the cavity which contained the 

 body of the animal at the moment of its death. The protection of a shell 

 seems to have rendered the presence of an ink-bag supurfluous. 



