MOUTHS AND JAWS 



107 



is carried still further, the upper jaw being completely fused 

 with the cranium, and the supporting element of the hyoid 

 arch is reduced to a mere vestige (Fig. 56A). 



In the class of Bony Fishes the primary upper and lower jaws 

 have become so much modified as to be scarcely recognisable 

 in the adult fish, but the early development of the mandibular 

 and hyoid arches throws considerable light on the manner in 

 which the changes have taken place. The skull commences 



pmx. 



pmx 



Fig. 46. CARTILAGINOUS AND BONY JAWS. 



A. Lateral view of skull of Spotted Dog-fish {Scyliorhinus sp.), X ^ ; b. The same 

 of Ten-pounder (Elops saurus),X^ ; c. Lower surface of skull of Pike (Esox 



lucius), X \. 



hr.i., first branchial arch; hym., hyomandibular ; /. jazv, lower jaw; mk.y 



Meckel's cartilage ; mx., maxillary ; pal., palatine ; pmx., praemaxillary ; 



piq., pterygo-quadrate ; smx., supra-maxillary. 



as a simple cartilaginous box, with a series of visceral arches 

 all more or less similar in form. As growth proceeds, the first 

 pair of these arches takes on the form of the jaws as seen in the 

 Dog-fish, and these are suspended from the cranium through 

 the intervention of the hyomandibular. Soon afterwards the 

 cartilages are replaced by bones, these being of two kinds, 

 cartilage bones which eat into and take the place of the original 

 cartilage, and dermal bones which arise as entirely new struc- 

 tures by the deposition of lime salts in the dermal layer of the 

 skin. It will be unnecessary to deal with the disposition and 



