s air-chambers, though the siphuncle passes through them,
have no direct connection with one another; and it is believed
that the animal has the power of slightly altering its specific
gravity, and thus of rising or sinking in the water by driving
additional fluid into the siphuncle or partially emptying it.
The _Orthoceras_ further agrees with the Pearly Nautilus in the
fact that the partitions or septa separating the different
air-chambers are simple and smooth, concave in front and convex
behind, and devoid of the elaborate lobation which they exhibit
in the Ammonites; whilst the siphuncle pierces the septa either
in the centre or near it. In the Nautilus, however, the shell is
coiled into a flat spiral; whereas in _Orthoceras_ the shell is
a straight, longer or shorter cone, tapering behind, and gradually
expanding towards its mouth in front. The chief objections to
the belief that the animal of the _Orthoceras_ was essentially
like that of the Pearly Nautilus are--the comparatively small
size of the body-chamber, the often contracted aperture of the
mouth, and the enormous size of some specimens of the shell.
Thus, some _Orthocerata_ have been discovered measuring ten or
twelve feet in length, with a diameter of a foot at the larger
extremity. These colossal dimensions certainly make it difficult
to imagine that the comparatively small body-chamber could have
held an animal large enough to move a load so ponderous as its
own shell. To some, this difficulty has appeared so great that
they prefer to believe that the _Orthoceras_ did not live in
its shell at all, but that its shell was an internal skeleton
similar to what we shall find to exist in many of the true
Cuttle-fishes. There is something to be said in favour of this
view, but it would compel us to believe in the existence in Lower
Silurian times of Cuttle-fishes fully equal in size to the giant
"Kraken" of fable. It need only be added in this connection that
the Lower Silurian rocks have yielded the remains of many other
Tetrabranchiate Cephalopods besides _Orthoceras_. Some of these
belong to _Cyrtoceras_, which only differs from _Orthoceras_ in
the bow-shaped form of the shell; others belong to _Phragmoceras_,
_Lituites_, &c.; and, lastly; we have true _Nautili_, with their
spiral shells, closely resembling the existing Pearly Nautilus.

Whilst all the sub-kingdoms of the Invertebrate animals are
represented in the Lower Silurian rocks, no traces of Vert