FISH IN GENERAL. 65 



by the accumulation of small calcareous granules, which 

 gives them a stippled appearance. Their cranium in form is 

 similar to that of other fish, and nevertheless of only one 

 inclosure, without sutures. Their face is very simple, with but 

 two bones in the palato-temporal arch ; the first descends from 

 the cranium, at the articulation of the jaws ; the other repre- 

 sents the upper jaw, and bears the teeth. The maxillary and 

 intermaxillary are reduced to vestiges. The under jaw has 

 also but one bone on each side (the articular), bearing the 

 teeth ; of the others, only one vestige is found concealed under 

 the skin of the lip. They have no opercular apparatus, but 

 the hyoidian and branchial structure is in great conformity 

 with the same parts in osseous fish. Sharks have besides, 

 opposite to the external attachment of each branchia, a slender 

 bone, a true vestige of rib. The branchial system is placed 

 further back than in osseous fishes, beneath the commence- 

 ment of the spine, whence, the humeral girdle is also further 

 back. This girdle is in rays and sharks of one piece; in 

 the former it attaches to long processes of the spine, and in 

 the latter it is quite free. On the sides of this circle are the 

 pectoral fins, attached by several pieces. The pelvis is 

 similarly a single transverse piece, not articulated to the 

 spine, but bearing on each side a blade to support the ventrals. 

 In the spine several vertebrae are soldered together, and there 

 are twice as many superior rings as vertebrae. The spinal 

 ribs, if any, are usually very small, excepting in sturgeons. 



This genus is also, as respects the branchiae, intermediate 

 between cartilaginous and osseous fishes. Several bones of 

 the head and shoulder are hard as stone, but not fibrous, yet 

 the spine is almost like that of lampreys. 



VOL. X. F 



