74 STUDIES ON THE ELASMOBRANCII SKELETON, 



prolongation forwards of the notochorcl, while its sides articulate 

 with the occipital condyles. 



In the caudal region of Sharks the haemal spines become 

 specially produced to afford support to the ventral half of the 

 broad caudal fin ; closely related to the dorsal surface of the 

 vertebrae in this region, but not continuous with the neural 

 arches and supporting the dorsal half of the fin are a series of 

 cartilages which, though resembling produced neural spines, 

 are not continuous with the neural arches, and do not agree 

 in number with the latter ; and in the trunk the dorsal and 

 anal fins possess cartilaginous supports which sometimes present 

 the appearance of modified neural and haemal spines. Frecpiently> 

 however, the skeleton supporting these unpaired fins is entirely 

 unconnected with the vertebral column, and, though in some 

 cases the correspondence in position of its elements with the 

 segments of the vertebral column seems to indicate some develop- 

 mental relationship with the latter, in others, the skeleton of 

 the fin becomes so modified by the formation of plate-like basal 

 cartilages that this apparent correspondence is no longer traceable. 



The cranium of the Plagiostomata is formed of an undivided 

 mass of cartilage, strengthened in many cases by the deposition 

 of superficial layers of bony matter. It consists, in essence, of a 

 cartilaginous case into the sides of which are incorporated the 

 cavities or capsules that serve to protect the three pairs of organs 

 of special sense, the ear, the eye, and the nose ; in the walls of 

 which are apertures for the egress of the cerebral nerves ; and 

 from which project certain processes which serve for the attach- 

 ment of muscles or for connection with the anterior visceral 

 arches. In the middle line behind is the aperture of the foramen 

 magnum, the plane of which is usually inclined from below and 

 behind upwards and forwards ; its lower lip is deeply excavated 

 in the Rays for the reception of the mesial process of the first 

 vertebra ; and on either side of this is one of the condylar surfaces 

 which are articulated with the lateral articular processes of the 

 first vertebra. In some sharks ( Notidanidce), the occipital region 

 presents certain characteristics which assimilate it to the vertebral 

 column ; in the middle above is a ridge seeming to prolong 



