78 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



of the zygomatic-postorbital bar, the temporal fossa and the orbit 

 may unite. 



One or another of these bones may disappear in some groups, either by 

 fusion or by complete dropping out. Occasionally they may obtain different 



FiG.(^) — Dorsal and ventral views of skull of young Sphenodon, after Howes and 

 Swinnerton. bs, basisphenoid; bo, basioccipital; eo, exoccipital;/, frontal; A, anterior 

 comu of hyoid; if, infratemporal fossa; j, zygomatic (jugal); mx, maxillary; «, nasal; 

 oo,|opisthotic;/', parietal; />/, postfrontal; />i, palatine; pm, premaxillary; />oo, postor- 

 bital;i|/>o/, postfrontal; ps, parasphenoid; pt, pterygoid; q, quadrate; qj, quadratojugal; 

 sf, supratemporal fossa; so, supraoccipital; sq, squamosal; v, vomer; cartilage dotted. 



•connexions and relations, as in the case of the quadrate in mammals (see ear 

 bones) so that the homologies are traced with difficulty. The complexity is 

 increased by the fusion of membrane bones and cartilage bones and by the 

 union of cranial bones with those of the visceral arches. 



In the lower jaw there 

 are no such extensive modi- 

 fications as in the upper. 

 At most Meckel's cartilage 

 gives rise by ossification to 



Fig. 77. — Reconstruction of developing jaw of , r • vi. i. ir 



Sceloporus, cartilage dotted; an, angulare; ar, ^WO bones m either half, 

 articulare (not yet ossified); CO, coronoid;d,dentary; Behind, at the articulation 

 g, antarticular or goniale; mk, Meckel's cartilage; r , . • ^ 1 



sa, surangulare; spl, splenial. 01 the jaw With the quad- 



rate, there is an articular 

 bone, while at the tip, at either side of the union (symphysis) of 

 the two halves of the jaw, there is rarely a mento-Meckelian bone. 

 The rest of Meckel's cartilage forms an axis around which the mem- 

 brane bones which form the definitive jaw are arranged (fig. 77). 



