THE PERCH 57 



The hinder end of the upper jaw is made up of a series of 

 paired bones which have ossified in or around the palatoquadrate 

 cartilage, which forms the embryonic upper jaw of the teleost 

 and the adult upper jaw of elasmobranchs. The quadrate bone is 

 a large triangular bone at the hinder end of the upper jaw on each 

 side ; with this bone the lower jaw articulates. The metapterygoid 

 is a large bone lying just above the quadrate and back of the orbit. 

 The pterygoid is an elongated bone which projects in front of the 

 quadrate. The mesopterygoid is a thin plate above the anterior 

 end of the pterygoid and forming a part of the roof of the mouth. 

 In front of the last two bones is the palatine, which helps to form 

 the roof of the mouth and bears teeth. 



Of these bones the quadrate and the pterygoids are ossifications 

 of the palatoquadrate cartilage ; the palatine is a membrane bone 

 which forms around the anterior end of this cartilage. 



Surrounding the orbit and forming its margin on all except the 

 dorsal side is a row of membrane bones called the suborbitals, 

 which are very easily lost in the cleaning. The most anterior 

 one is a large bone between the orbit and the maxilla, the lachry- 

 mal, on the surface of which will be seen radiating canals belong- 

 ing to the lateral-line system of sensory canals. 



The Hyoid Arch. The dorsal half of this arch forms the sus- 

 pensorium of the jaws. It is formed by the hyomandibular bone 

 and the symplectic bone. The former is a large bone which ex- 

 tends from the cranium ventrally, posterior to the metapterygoid, 

 to the symplectic; the latter is a small bone which lies just 

 dorsal to the hinder end of the quadrate. The jaws are thus joined 

 with the cranium. 



The ventral half of the arch is formed by a row of bones, on 

 each side, which extend ventrally from the junction of the sym- 

 plectic and the hyomandibular and are called collectively the 

 hyoid bones. These bear the branchiostegal rays. In the middle 

 area between the right and left sides of the hyoid arch is the 

 basihyal bone, which supports the tongue. 



Articulating with both the dorsal and ventral halves of the 

 hyoid arch on each side are bony plates and rods which support 

 structures guarding the openings of the gill slits; these are the 



