SKELETON 79 



These are, at most, as follows: (i) a dentary which surrounds the 

 Meckelian in front and usually bears teeth; (2) a splenial on the 

 inner side, behind the dentary and frequently bearing teeth; (3) an 

 angidare on the lower side, usually extending back to the hind end 

 of the jaw; (4) a surangulare on the outer posterior part of the jaw; 

 (5) a coronoid on the upper side, affording attachment for the 

 muscles which close the jaws; and (6) a goniale (antarticular or der- 

 marticulare) on the medial and ventral sides of the articulare, with 

 which it usually fuses. This whole series is present in few verte- 

 brates, dentary, splenial and angulare being the most constant. 



In the hyoid and branchial arches ossification occurs to a greater 

 or less extent, the resulting cartilage bones having the same names as 

 the corresponding cartilages. There are never any membrane bones 



Fig. 78. — Skull of Helerodontus fransisci, after Daniel. A, hyomandibular; I, 

 labial cartilages; m, Meckel's cartilage (lower jaw); n, nasal capsule; op, optic pedicle; 

 Pq, pterygoquadratc; sc, spiracular cartilage. 



in this region. In the teleosts the hyomandibular ossifies as two 

 bones, a dorsal hyomandibular and a lower symplectic which con- 

 nects with the quadrate. There is, however, a considerable amount 

 of union between the various arches in the adults of all tetrapoda, 

 where the branchial respiration is lost and the arches have to 

 assume other functions than the support of gills. 



The mode of suspension of the jaws varies. In a few elasmo- 

 branchs the pterygoquadrate articulates directly with the cranium 

 (amphistylic) (fig. 78) ; in others it is suspended by ligament and 

 by the interposition of the hyomandibular between the otic capsule 

 and the hinder end of the jaw (hyostylic, fig. 84); while in all 

 groups above the fishes the pterygoquadrate is more or less com- 

 pletely fused with the cranium (autostylic). 



