xm 



PHYLUM CHORDATA 



359 



A 



Pmx 



America. The Alligators are represented in North America by 

 one species and in China by another. The true Crocodiles occur 

 widely distributed over Africa, Southern Asia, the northern parts 

 of Australia and tropical America, while the Gavial occurs only in 

 certain Indian and Burmese rivers. 



Geological Distribution. The Squamata are geologically the 

 most recent of the existing orders of Reptiles. The earliest fossil 

 remains of 

 Lizards have 

 been found in 

 beds belonging 

 to the Jurassic 

 and Cretaceous 

 periods; but 

 most of the 

 families are not 

 represented 

 earlier than the 

 Tertiary. All 

 the known fossil 

 remains of 

 Snakes, except 

 one imperfectly 

 known form from 

 the Cretaceous, 

 have been found 

 in deposits of 

 Tertiary age. 

 The Rhynchoce- 

 phalia are much 

 more ancient, 

 being represented 

 in deposits as old 

 as the Permian 

 by a genus- 

 Pa I CB o h alter ia 

 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 Rhyncho- 

 cephalians have been found in Triassic and in Tertiary strata. The 

 order Chelonia was represented from the Triassic period onwards. 

 Of the extinct forms 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 vertebraa 

 and ribs. The Crocodilia date back as far as the Trias. The most 

 primitive of the fossil forms (Fig. 1021) had the internal nares 



z 2 



FIG. 1021. Skull of Belodon. A, from above ; B, from below. 

 A, orbit ; Bo, basi-occipital ; Ch, internal nares ; D, pre-orbital 

 fossa ; Exo. exoccipital ; Fr. frontal ; Ju. jugal ; La. lacrymal ; 

 MX. maxilla ; n. external nares ; Na. nasal ; Pa. parietal ; PI. 

 palatine ; Pmx. premaxilla ; For. post-orbital ; Prf. pre-frontal ; 

 Pt. pterygoid ; Qu. quadrate ; S, lateral temporal fossa ; S' t 

 superior temporal fossa ; Sq. squamosal ; Vo. vomer. (Prom 

 Zittel.) 



