278 OBSERVATIONS ON THE ANIMALS 
Such are the analogies: the differences are sufficiently remarka- 
ble, and refer chiefly to the walls of separation or septa between the 
chambers, the siphuncle or tube running through the chambers, and 
the peculiar methods adopted to strengthen the resisting power of 
the empty cells. 
The first difference is that relating to the septa. In the Nauti- 
lacea these are simple plates, concave outwardly, and presenting a 
nearly regular curve in their intersection with the shell. On the 
other hand, they are in the Ammoneata, always more or less circular 
in their form, and the line of intersection varies from a kind of zig- 
zag, to as complicated a curve as can be imagined to exist. There 
is, thus, in the species of this family, a contrivance by which each in- 
ternal transverse plate, presents a much longer line for the shell 
to press against, or, if you will, offers a much greater number of 
points of resistance than in the more simple-formed Nautilacea. 
The next point of difference to be attended to, is in the siphuncle, 
and it is one both of position and magnitude. In the genera of the 
first family, this important part is sometimes ventral, or on the in- 
ner margin, more frequently central, and is very rarely observed to 
approach the dorsal or outer margin. On the other hand, it is al- 
most always very near the dorsal margin in the Ammoneata, and 
sometimes is actually placed outside, in a channel opened for it, and 
projecting from the back of the shell in the shape of a keel. The 
difference in size is not so constant; but it is not, certainly, too 
much to assert, as a general rule, that it bears a very much smaller 
proportion to the area of a section across the chamber in Ammonites 
and their congeners, than in the Nautiloid forms. The siphuncle, 
then, is, on the whole, more undefended, and apparently less im- 
portant, in the former than in the latter. 
Lastly, there is the difference of plan resorted to in the two fami- 
lies to accomplish the same purpose, that of giving a degree of 
strength not found in other shells. In Nautilus, and generally in 
the Nautilacea, the most simple means are employed. The shell is 
tolerably thick, the whorls often successively envelope each other, 
and the surface exposed forms a tolerably strong single arch. Not 
so is the case with most of the Ammoneata. The shell is usually 
extremely thin, even in the very large species; the whorls do not, 
or at least very rarely, envelope ; and the spiral disc, being flat- 
tened, as it often is in the transverse section, is frequently elliptical, 
and sometimes almost angular. But now for the compensation. 
Instead of a simple arch, we have one which is fluted ; and we find 
a set of contrivances, consisting of ribs and domes, with transverse 
es 
