—="~S i —-” - 
INHABITING MULTILOCULAR SHELLS. ~ 277 
cone, which is the simple and ordinary form of thé Baculite. Both 
are sometimes ribbed, though the latter rarely ; and in both the 
edges of the septa are very complicated. The shells of both, too, 
are commonly rather thin, the siphuncle not large, and the com- 
plete fossil, especially of the Baculite, extremely rare. 
So much for the external form of the shells referred to the great 
natural family Ammoneata ; a family which deserves well the con- 
sideration of the paleontologist, since of all others it is, as we have 
already remarked, the most widely spread, the most characteristic of 
several formations, and which is, moreover, just so nearly allied to 
living types as to excite our curiosity, and raise our hopes that some- 
thing may sooner or later appear, some new discovery or closer ob- 
servation made, by which our speculations may be tested, and their 
truth or falsity proved. 
With a full knowledge, then, of the danger of too rash generaliza- 
tions, and wishing it to be clearly understood that our theories and 
attempted analogies are rather thrown out with a view to excite 
inquiry than with any expectation of satisfying doubts, or still more 
of deciding dogmatically on any disputed point, we will proceed to 
the consideration of the following queries, viz. how far the analogies 
traceable in the shells of the Nautilacea and Ammoneata are indica- 
tive of analogous habits, and to what change of habits of the animal 
the alteration of structure in the two families may most probably 
refer. 
Now, that the subject may be more perfectly understood, let us 
here just repeat, in as few words as possible, the most important 
both of the analogies and differences, because, when they are thus 
put in apposition, our plan of considering the matter will be more 
clear to the reader, and its value, however small, more truly appre- 
ciated. 
The shells, then, of the two families, are, in all cases, multilocular, 
the chambers being filled with air during the life of the animal, and 
forming then a mass not very different in weight from the weight of 
an equal mass of water. In other words, the shell would float of 
itself under the ordinary conditions. Besides this, the walls of se- 
paration of the chambers are always pierced with a hole larger or 
smaller, through which a tube has passed, communicating with the 
interior of the animal. There is, in all the species, considerable 
regard paid to the general strength of the shell in some way or 
other, more especially where the animal is no longer present, for the 
empty cells are stronger than the outer one, which is, in most 
the only one inhabited. 
