STRUCTURE OF SKULL. 135 



they will readily fall out in a dried skull ; whereas 

 those of lizards, which vary greatly in form, are in- 

 variably united by solid bone with the edges or 

 sides of the jaws, without any separate socket. More- 

 over, in a crocodile's skull, there is a bar of bone running 

 backwards from the lower border of the eye-socket, or 

 orbit (Fig. 44, 0), to join the condyle with which the 

 lower jaw articulates. This bar is seen in Fig. 44, below, 



and to the 

 left of the 

 letter O, 

 and also 



occupying 

 the same 

 relative 



Fra. 44. Side view of the Skull of a Crocodile ; position in 

 O, eye-socket or orbit. Fig. 46. It- 



will further 



be apparent from the latter figure that in a crocodile's skull 

 there are two parallel bars running backwards from behind 

 the orbit, of which the upper one is the stouter. Now 

 in a lizard's skull, only the uppermost of these two bars 

 is present ; and we thus have a second important distinc- 

 tion between a crocodile and a lizard. A still more impor- 

 tant difference occurs, however, in the under part of the 

 skull. Thus, whereas in a lizard the external nostrils 

 open directly through the palate into the front part of 

 the mouth, in a crocodile the bones of the palate develop 

 a kind of flooring beneath its roof, and thus form a closed 

 passage by which the internal or posterior nostrils are 

 brought to the very hinder extremity of the skull ; this 

 remarkable peculiarity being well exhibited in Fig. 45, A, 

 where N indicates the internal nostrils. The small round 

 aperture seen in the front of the palate in both A and B 

 is closed during life with membrane, and thus prevents 

 any communication between the external nostrils and the 

 front of the mouth. The object of this peculiar arrange- 

 ment is to enable the animal to breathe when its mouth 

 is open under water, and the nostrils are alone in the air ; 

 this being effected by the closing of the back of the mouth, 



