CLASSIFICATION 37 
there is a picture in the possession of the Geological 
Society of London representing the fossil head of a 
reptile (Ichthyosaurus) executed with the fossil sepia 
from a Belemnite preserved in the same strata. 
The gills of the Cephalopoda are aspidobranch in 
type, and either four or two in number: the class 
is consequently divided into Tetrabranchia and 
Dibranchia. 
Among the Cephalopoda we meet, for the first 
time in the Mollusca, with internal structures of 
great import—namely, cartilages—which are espe- 
cially developed in the head. In Nawtilus there is 
an H-shaped cartilage, which supports the ventral 
portion of the nerve centres, two of its branches 
extending to the base of the funnel. In the Dibran- 
chia a cartilage completely invests the central nervous 
system, the cesophagus passing through it. Different 
Cephalopoda have additional cartilaginous pieces in 
other parts of the body, such as the bases of the fins 
and arms, at the base of the neck (when the mantle 
is not fused to the head), at the internal extremities 
of the retractor muscles of the head and funnel, and 
even in the two branchial lamelle. 
The shell, as seen in its fullest (external) develop- 
ment in the Nautilus and its fossil relations, as well 
as in the Ammonites, resembles that of the Gastro- 
poda in consisting of a single conical tube. In the 
earliest kinds of Nautilus (Orthocervas) the shell is 
quite straight, in some others it is curved; but in 
