126 Basic Structure of Vertebrates 



bone, a middle pterygoid (or pterygoids), and a posterior quadrate 

 which is the upper member of the joint between the upper and lower 

 jaws. In vertebrates other than fishes, the cartilaginous upper jaw is 

 more or less deficient anteriorly, and the deficiency is compensated for 

 by development of dermal bones which, according to their location, 

 are named palatine or pterygoid. The quadrate is invariably de- 

 veloped as a cartilage bone. In the middle or pterygoid region of the 

 jaw, one or more pterygoid bones develop, partly cartilaginous and 

 partly dermal in origin. Except in some fishes, the palatine bones are 

 probably entirely dermal. Closely parallel and adjacent to the palato- 

 pterygo-quadrate series of bones is developed a second series (Fig. 120, 

 26-29) consisting entirely of dermal bones, named, beginning ante- 

 riorly, the premaxilla, maxilla, jugal (called "malar" in mammals), 

 and the quadratojugal, which adjoins the quadrate. The premaxilla 

 and maxilla are the tooth-bearing bones in mammals, but in other 

 vertebrates teeth may be present not only on them but also on various 

 other dermal bones of the jaw and roof of mouth. 



In the lower jaw the only cartilage bone formed is at the posterior 

 end of the mandibular cartilage — the articular (Fig. 120, 31) which, 

 except in mammals, is the lower member of the joint between lower 

 and upper jaws. In all bony skulls the mandibular cartilage becomes 

 completely invested by dermal bones. Chief and most anterior of them 

 is the dentary, the tooth-bearing bone. At the posterior region of the 

 lower jaw may be a ventral angular, an external supra-angular 

 (or surangular), an internal splenial (sometimes bearing teeth), and 

 a prominent dorsal coronoid, to which are attached the strong muscles 

 which close the jaws. In mammals each lateral half (ramus) of the 

 lower jaw is a single dermal bone articulating with the squamosal, 

 which functionally replaces the quadrate. 



The hyoid and branchial arches, except in the sharklike fishes 

 and a few others, undergo more or less ossification, reaching a maxi- 

 mum in the "bony fishes." But ordinarily no dermal bone is formed 

 in relation to these arches. In adult vertebrates other than fishes, 

 these arches are greatly reduced and modified. 



In fishes other than the sharks and their allies, the external aper- 

 tures of the gill-chambers are covered by a thin plate, the operculum, 

 whose free posterior edge permits escape of water from the gill-cham- 

 bers. The operculum is a fold of skin supported by extensive sheets 

 of dermal bone. There are at most four of these bones (Fig. 120, 45-4#), 

 named, according to their relative positions, opercular, preopercu- 

 lar, subopercular, and interopercular. These opercular bones are 

 peculiar to fishes. 



