THE SKELETON IN AMPHIBIA. THE SKULL. 173 



THE SKULL'. 



CRANIUM AND MANDIBLE. 



In the Amphibian skull there are as a rule far fewer bones 

 than in the skull of bony fish. The primordial cartilaginous 

 cranium often persists to a great extent. Only quite a few 

 ossifications take place in it ; namely in the occipital region 

 the exoccipitals, further forwards the pro-otics, and at the 

 boundary of the orbital and ethmoidal regions the sphen- 

 ethmoid. The basi-occipital and basisphenoid are never 

 ossified. As in Mammalia there are two occipital condyles 

 formed by the exoccipitals. 



Large vacuities commonly occur in the cartilage of both 

 floor and roof of the primordial cranium. These are roofed 

 over to a greater or less extent by the development of mem- 

 brane bone. Thus on the roof of the cranium there are paired 

 parietals, frontals, and nasals, and on its floor are paired 

 vomers, and a median unpaired parasphenoid. 



In all living forms the parietals meet and there is no 

 interparietal foramen, though this exists in Labyrintho- 

 donts. 



The palato-pterygo-quadrate bar is united at each end with 

 the cranium, but elsewhere in most cases forms a wide arch 

 standing away from it. The suspensorium is, as in Dipnoi 

 and Holocephali, autostylic. The palato-pterygo-quadrate bar 

 sometimes remains entirely cartilaginous, sometimes its pos- 

 terior half is ossified forming the quadrate. In connection 

 with it a number of membrane bones are generally developed, 

 viz. the maxillae, premaxillae, palatines, pterygoids, quadrato- 

 jugals, and squamosals. The pterygoids are, however, some- 

 times partially formed by the ossification of cartilage. The 



1 See many papers by W. K. Parker published in the Phil. Trans, of 

 the Koyal Soc. 



