in the Fish Gallery of the Indian Museum. 23 



The skull is an unsegmented cartilaginous capsule, of 

 which the roof is often in places incomplete, and to it 

 the cartilages that protect the organs of special sense 

 are firmly attached, — the snout, or parts enclosing the 

 organ of smell, being often enormously developed. The 

 jaws, which are much less complicated than they are in 

 the bony fishes,. and the hyoid bone, are also of carti- 

 lage ; as also are the branchial arches proper, which 

 further differ from those of the bony fishes (1) in being 

 suspended from the anterior part of the " backbone," 

 (2) in having cartilaginous branchial rays for the support 

 of the gill-chambers, and (3) in not having a gill-cover. 



The backbone consists of a chain of cartilaginous 

 vertebrae and is prolonged into the upper lobe of the 

 caudal fin, forming a hcterocercal tail. 



The appendicular skeleton consists of two cartilagin- 

 ous arches — one for the pectoral fins the other for the 

 ventral fins — to which plates of cartilage are attached, 

 and from these the cartilaginous fin-rays radiate. 



The typical cartilaginous skeleton, above briefly described is not found, 

 unmodified, in all the members of the large Order (of Sharks and Rays) to 

 which the Dog-fish belongs. For instance, there are Sharks in which the 

 vertebrae, and even some of the bones of the head, are more or less calcified ; 

 there are Rays in which the vertebra? are partly ossified ; and there is one 

 small section of the Order in which rudimentary gill-covers are present. 



The essential difference between the skull of a Shark and that of any of the 

 higher Vertebrates is, that in the latter the primitive skull is not only ossified, 

 but also becomes roofed-in by large bones (dermal bones) formed in the original 

 integumentary covering of the skull. In the Shark the primitive skull remains 

 unossified and unconnected with any ossifications in its integumentary cover- 

 ing. 



It has been stated that the rays of the paired fins of 

 Fishes are not connected with any stem representing 

 the axis of the limb of higher Vertebrates. An excep- 

 tion to this statement must now be made. 



