320 USE OF CHAMBERED SHELLS. 
food on which they are respectively destined to 
feed, so we may conclude, from the resemblance 
of the fossil beaks, or Rhyncholites, (PI. 31, Fig. 
6 — 1 1), to the calcareous portions of the beak of 
the Cephalopod, inhabiting the N. Pompilius, 
that many of these Rhyncholites were the beaks 
of the cephalopodous inhabitants of the fossil 
shells with which they are associated ; and that 
these Cephalopods performed the same office in 
restraining excessive increase among the Crus- 
taceousand Testaceous inhabitants of the bottom 
of the Transition and Secondary seas, that is 
now discharged by the living Nautili, in con- 
junction with the carnivorous Trachelipods.* 
Assuming, therefore, on the evidence of these 
analogies, that the inhabitants of the shells of the 
fossil Nautili and Ammonites were Cephalopods, 
of similar habits to those of the animal which 
constructs the shell of the N. Pompilius, we shall 
next endeavour to illustrate, by the organization 
and habits of the living Nautilus, the manner in 
which these fossil shells were adapted to the use 
of creatures, that sometimes moved and fed at 
the bottom of deep seas, and at other times rose 
and floated upon the surface. 
The NautiU (see PL 31. Fig. 1. and PI. 32. 
Figs. 1. 2.) constitute a natural genus of spiral 
discoidal shells, divided internally into a series 
* See p. 250. 
