ANATOMY OF VERTEBRATES. 



81 



65 



arch consists of a basihyoid and two simple ceratohyoid carti- 

 lages ; the stylohyal is ligamentous, as in the Squatina. Short 

 cartilaginous rays diverge from the ceratohyal to support the 

 branchiostegal membrane, or hyoid fin. The scapular arch, 

 which we shall find normally articulated with the occiput in 

 osseous fishes, is attached thereto, at a little distance behind the 

 head, by ligament and muscles in the Sharks, fig. 30, 51 : from 

 this arch, also, cartilaginous rays, ib. k, I, immediately diverge for 

 the support of a radiated appendage or fin — the homotype of the 

 tympanic or opercular fin. 



The capsules of the special organs of sense are all cartilaginous : 

 that of the ear is involved in the lateral 

 walls of the cranium ; that of the eye is 

 articulated by a cartilaginous pedicle with 

 the orbit ; and that of the nose, figs. 30 and 

 63, b, is overarched by the nasal processes 

 of the epicranial cartilage, ib. a, and is 

 completed below by membrane. At the 

 summit of the occiput in Carcharias and 

 some other sharks may be seen two closely 

 approximated oval ' fenestras,' which lead 

 to the acoustic labyrinth, and are covered 

 by skin in the recent fish. 



Amongst the stranger forms in which 

 special developement radiates, in diverging 

 from that stage of the common vertebrate 

 route attained by the Plagiostomes, may 

 be noticed the lateral transverse elonga- 

 tions of the orbital processes, supporting 

 the eyeballs at their extremity, and giv- 

 ing the peculiar form to the skull of 

 certain Sharks, thence called 'Hammer- 

 headed' {Zygoma). In the < Saw-fish' 

 (Pristis), the rostrum, fig. 65, is produced 

 into a long, flat, plate, having a row of 

 tooth-like bodies implanted in sockets along 

 each maroin. The walls of these sockets 

 and the midpart of the rostrum are ossified. 

 The proper jaws and teeth a have the usual inferior position in the 

 Sharks. In the Eagle-ray (Myliobates) a cartilage is attached to 

 the anterior prolonged angle of the great pectoral fin, and con- 

 nects it with the fore part of the cranial (internasal) cartilage ; 

 it supports a number of branched and jointed cartilaginous rays, 



vol. i. G 



Mouth and rostrum of Saw-flsh 

 (Pristis). 



