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 Nautilus 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 oesophagus 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 lamellae. 



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 (Orthoceras) the shell is 

 quite straight, in some others it is curved ; but in 



