THE SKELETON IN AMPHIBIA. THE SKULL. 179 



the bones which bound the mouth cavity. The cranium is 

 very hard, and is covered by a complete bony roof formed 

 mainly of the exoccipitals, parietals, frontals, prefrontals, 

 nasals and premaxillae. The nasals and premaxillae are 

 sometimes ossified continuously. There is a median unpaired 

 ethmoid whose dorsal end appears at the surface wedged in 

 between the frontals and parietals. The bone generally 

 regarded as the squamosal 1 is very large, and it and the 

 maxilla generally together surround the orbit, which, in 

 .Epicrium, has in it a ring of bones. The palatines form long 

 tooth-bearing bones fused with the inner sides of the maxillae; 

 they nearly surround the posterior nares. 



The quadrate bears the knob, and the angular the cup 

 for the articulation of the mandible, a very primitive feature. 

 The mandible is also noticeable for the enormous backward 

 projection of the angular. 



ANURA. In Anura the skull is very short and wide 

 owing to the transverse position of the suspensorium. There 

 is often a small ossification representing the quadrate. Some- 

 times as in Hyla and Alytes there is a fron to-parietal 

 fontanelle. 



As compared with the skull in Urodela the chief charac- 

 teristics of the skull of Anura are : 



1. the presence of a sphenethmoid, 



2. the union of the frontals and parietals on each side, 



3. the occasional occurrence of small supra- and basi- 

 occipitals, 



4. the backward growth of the maxilla and its connection 

 with the suspensorium by means of the quadratojugal, 



5. the dagger-like shape of the parasphenoid, 



6. the occurrence of a definite tympanic cavity, 



1 Perhaps this bone includes supra-orbital and postorbital elements. 



122 



