244 THE VERTEBRATE SKELETON. 



presenting a number of striking characteristics, some of the 

 more important of which are : 



1. All the bones except the mandible, hyoid, and columella 

 are fir inly united by interlocking sutures. In spite of this, 

 however, growth of the whole skull and of the component 

 bones goes on continuously throughout life, this growth being 

 especially marked in the case of the facial as opposed to the 

 cranial part of the skull. 



2. All the bones appearing on the dorsal surface are 

 remarkable for their curious roughened and pitted character ; 

 this feature is prominent also in. many Labyrinthodonts. 



3. The size of the jaws and teeth is very great. 



4. The mandibular condyle is carried back to some dis- 

 tance behind the occipital condyle. 



5. The occipital plane (see p. 386) of the skull is vertical. 



6. The length of the secondary palate is remarkably great, 

 and the vomer takes no part in its formation. 



7. The posterior nares are placed very far back, the nasal 

 passages being as in mammals separated from the mouth by 

 the long secondary palate. 



8. There is a complicated system of Eustachian passages 

 communicating at one end with the tympanic cavity and at 

 the other end with the mouth cavity. 



9. The interorbital septum is mainly cartilaginous, the 

 presphenoidal and orbitosphenoidal regions remaining un- 

 ossified. 



The skull is divisible into three parts : 



(1) the cranium, (2) the lower jaw, (3) the hyoid. 



The cranium may again for purposes of description be 

 divided into :-<- 



1. the cranium proper or brain case ; 



2. the bones connected with the several special sense 

 organs ; 



3. the bones of the upper jaw, and suspensorial apparatus. 



