No trace of the true Cuttle-fishes has yet
been found in Lower Silurian deposits; but the Tetrabranchiate
group is represented by a great number of forms, sometimes of
great size. The principal Lower Silurian genus is the well-known
and widely-distributed _Orthoceras_ (fig. 55). The shell in this
genus agrees with that of the existing _Pearly Nautilus_, in
consisting of numerous chambers separated by shelly partitions
(or septa), the latter being perforated by a tube which runs the
whole length of the shell after the last chamber, and is known
as the "siphuncle" (fig. 56, s). The last chamber formed is the
largest, and in it the animal lives. The chambers behind this
are apparently filled with some gas secreted by the animal itself;
and these are supposed to act as a kind of float, enabling the
creature to move with ease under the weight of its shell. The
various 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.
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