SKELETON. 71 



groups vacuities or fossae appear in the postero-lateral parts, these being 

 bounded by bars or arcades" of bone. At most there may be three of 

 these fossae. The more lateral of these, the infratemporal fossa 

 (fig. 68), is bounded laterally by the zygomatic and quadratojugal, 

 while on the inner side it is separated from the supratemporal fossa 

 by a squamoso-postorbital arcade. The posttemporal fossa lies 

 between parietal, supratemporal and occipital bones. Occasionally 

 only the infratemporal fossa is present, or, by disappearance of the inter- 

 vening arcade, infra- and supratemporal fossae may unite in a single 

 temporal fossa. Lastly, by the breaking down 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 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 modifications as in the 

 upper. At most Meckel's cartilage gives rise by ossification to two 

 bones in either half. Behind, at the articulation of the jaw with the 

 quadrate, there is an artic- 

 ular bone, while at the 

 tip, at either side of the 

 union (symphysis) of the 

 two halves of the jaw, there 



is rarely a mentO-Meck- FIG. 69. Reconstruction of developing jaw of Scele- 



porus, cartilage dotted; letters as in fig. 68. 



ehan bone. The rest of 



Meckel's cartilage forms an axis around which the membrane bones 

 which form the definitive jaw are arranged. 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 angulare on the lower 

 side, usually extending back to the hind end of the jaw; (4) a supran- 

 gulare 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 (dermarticulare) on the medial and ven- 

 tral sides of the articulare, with which it usually fuses. This whole 

 series is present in few vertebrates, dentary, splenial and angulare 

 being the most constant. 



