The Skeleton of the Fish 



43 



The upper jaw of the shark is formed from the anterior por- 

 tion of tne palatine bones, which are not separate from the 

 quadrate, the whole forming the palatoquadrate apparatus. In 

 the himsera and the dipnoans this apparatus is solidly united 

 with the cranium. In these fishes the true upper jaw, formed 

 of maxillary and premaxillary, is wanting. 



The Lower /aw.— The lower jaw or mandible is also com- 

 plex, consisting of two divisions or rami, right and left, joined 

 in front by a suture. The anterior part of each ramus is formed 

 by the dentary bonej^^o), which carries the teeth. Behind this 

 is the articular bone (28), which is connected by a joint to the 



Fig. 27. — Lower jaw of Amia calm (Linnajus), showing tlie gular plate. 



quadrate bone (19). Xt the lower angle of the articular bone 

 is the small angular bone (29). In many cases another small 

 bone, which is called splenial, may be found attached to the inner 

 surface of the articular bone. This little bone has been called 

 coronoid, but it is doubtless not homologous with the coronoid 

 bone of reptiles. In a few fishes, Amia, Elopida, and certain 

 fossil dipnoans, there is a bony gular plate, a membrane bone 

 across the throat behind the chin on the lower jaw. 



The Suspensorium of the Mandible. — The lower jaw is at- 

 tached to the cranium by a chain of suspensory bones, which 

 vary a good deal with different groups of fishes. The articular 

 is jointed with the flat quadrate bone (19), which lies behind 

 the pterygoid. A slender bone passes upward (18) under the 

 preopercle and the metapterygoid, forming a connection above 



