INHABITING MULTILOCULAR SHELLS. 981 
mains extremely perfect, which we know, as far as analogy can 
make us certain, could not have lived at extreme depths. The same 
may be said of the chalk formation, with regard to the deposition of 
mud that it must have formed ; and it seems pretty certain that ani- 
mal life, for the most part, ceases to exist at any thing like deep 
water, just as on high mountains there are very few species which 
live and perform their functions. 
- It seems, then, not unreasonable to suppose that, since Ammonites 
and the allied genera are better adapted to resist regular pressure, 
but are more exposed to accidental injury than the Nautilacea— 
since, too, their siphuncle is generally smaller, and not so advan- 
tageously placed as in that family, but sometimes a mere thread, 
totally incapable of being employed as a means by which the animal 
could alter its depth in the sea—since, too, the Ammonites seem to 
have been abundant during the deposition of strata of calcareous 
mud—putting all these matters together, may we not venture to 
suggest that the animals of this family were, for the most part, the 
constant inhabitants of the bottom of the ocean in moderate depths, 
living there in the thick mud upon the numerous shell-fish and shell 
animals of all kinds, which we know to be very plentiful in such 
places ? 
The difference of habits, then, that we conceive to have existed, 
amounts to this: that while the Nautilus and its congeners were 
capable, probably, of coming to the surface, and even of feeding 
there occasionally, though upon the approach of danger they would 
immediately sink by means of their large siphuncle, the Ammoneata 
generally were more confined to the bottom, burrowing, perhaps, 
more or less, in the soft mud, and seldom wandering from their or- 
dinary habitats while they could subsist.quietly on the nourishment 
they found there. 
Of course, this general conclusion is not expected to include every 
particular case: there are Ammonites with large siphuncles, and 
Nautilacea with small ones. The genus Endosiphonites approxi- 
mates to Ammonites in external form and the existence of ribs on 
the shell, although it has a large siphuncle ; while, on the other 
hand, many Goniatites resemble Nautilus so closely in every thing 
but the siphuncle, that, were it not for the almost total absence of 
that organ, we should find an example of analogy to an unusual 
extent. Still the exceptional cases are not more numerous than in 
other branches of natural history, where a great majority of known 
facts is always considered sufficient foundation for theorizing. 
If it is admitted, then, that the structure in the great family 
VOL. IX., NO. XXVI, 36 
