XIII 



PHYLUM CHORDATA 



343 



Cretaceous, have been found in deposits of Tertiary age. The 

 Rhjnchocephalia are much more ancient, being represented in 

 deposits as old as the Permian by a genus Palseohatteria which, 

 though differing in some respects from the living Hatteria, is 

 sufficiently near it to be looked upon as a member of the same 

 order: and other extinct Rhynchocephalians have been found 

 in Triassic and in Tertiary strata. The order Chelonia was repre- 

 sented from the Triassic period onwards. Of the extinct forms 



Pmx 



Prf 



FIG. {151. Skull of Belodon. A, from above ; B, from below. A, orbit ; J3o, basi-occipital ; Ch. 

 internal nares ; D, pre-orbital fossa ; Exo. exoccipital ; Fr. frontal ; Ju. jugal ; La. lacrymal ; 

 MX. maxilla ; Na. nasal ; Pa. parietal ; PI. palatine ; Pmx. pre-maxilla ; Par. post-orbital ; 

 Prf. pre-frontal ; Pt . pterygoid ; Qu. quadrate ; S, lateral temporal fossa ; S', superior tem- 

 poral fossa : X'/. sqnamosal; Vo. vomer. (From Zittel.) 



one group the Athecata differs from the living Chelonia in 

 having the carapace incompletely developed, entirely composed of 

 dermal elements, and quite separate from the vertebrae and ribs. 

 The Crocodilia date back as far as the Trias. The most primitive 

 of the fossil forms (Fig. 951) had no palatine plates separating off 

 a posterior nasal passage from the cavity of the mouth and had the 

 external nares situated towards the middle of the snout. Later 

 forms (post-Triassic) had palatine plates developed from the pre- 



