: INHABITING MULTILOCULAR SHELLS. 279 
plates supporting them along a wavy line sometimes eight or ten 
times as long as the perimeter of a section, and all this without giv- 
ing to the shell an increase of weight at all inconsistent with the 
supposed habits of the animal. It should not, however, escape 
notice, that the additional strength would only avail against regular 
pressure, while the diminished thickness would render the shell 
more susceptible of injury from accidental causes, than the more 
simple covering of the Nautilus. 
These being the points of difference, let us now consider their 
comparative importance, and then proceed with a few speculations, 
with which this part of our subject will be concluded. 
Referring, then, to a former part of this article, where it was 
endeavoured to give an outline of the comparative anatomy of the 
Nautilus, it will appear that, since in that animal the siphuncle 
passes between important viscera and enters the pericardium, we are 
at liberty to conclude that it serves very necessary purposes in the 
animal economy ; for otherwise, according to all principles of ana- 
logy, there would not be so much care taken for its preservation, nor 
would it be connected with such vital organs. Also the position, as 
wellas the magnitude, of that curious tube, is a question of some 
importance ; for it could not pass through the same viscera, or indeed 
into the pericardium at all, when ventrally or dorsally situated, 
without making rather a sharp turn upwards or downwards. Again, 
since the mantle in the Nautilus covers that part of the body of the 
animal within the shell, and is attached by two lateral muscles only, it 
is fair to suppose that the connexion with the siphuncle was not 
entirely useless in assisting to make a central fastening, without 
which the two lateral ones could hardly be sufficient. Now, in all 
the forms of which the Ammonite is the type, there was a more or 
less irregular shape, to which the mantle could more easily fasten 
itself than to the smooth cup which forms the outer chamber of the 
Nautilus; and the alternate indentations and projections in the 
septum, together with those in the ribs and tubercles of the shell, 
must have kept the shell well attached without much assistance 
from the siphuncle. 
Taking into consideration, then, the uses of this tube, both as a 
means of altering the specific gravity of the whole mass, and also 
as a string by which the animal inhabitant was steadied in its abode 
—remembering that in the Nautilus and its congeners it is, almost 
without exception, tolerably capacious, and could have entered the 
body without much difficulty and at small disadvantage, while in 
the Ammoneata the size is diminished, and the mechanical disad- 
