PHYLUM CHORDATA 



371 



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

 Rhynchocephalia are much more ancient, being represented in 

 deposits as old as the Permian by a genus Palceohatteria 

 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 

 one group the Athecata differs from the living Chejonia in 

 having the carapace incompletely developed, entirely composed of 

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

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

 of the fossil forms (Fig. 1010) had the internal nares situated in 

 front of the palatines, while the external nares were situated 

 towards the middle of the snout. Later forms (post-Triassic) had 

 palatine plates developed from the premaxillae, the maxillas, and 

 the palatines; and some resembled the living members of the 

 order in having such plates developed also from the pterygoids ; 

 all had the external nares situated towards the end of the snout. 

 Those in which the palatine plates of the pterygoids were absent 

 had usually amphicoelous vertebra. Some of the fossil Crocodiles 

 reached an immense size. 



4. EXTINCT GROUPS OF REPTILES. 



THEROMORPHA. 



TILE Theromorpha are a very extensive and varied group of fossil Reptiles 

 which exhibit remarkable points of resemblance to the Amphibia (Stegocephala) 

 on the one hand, and to the lower Mammals on the other. They all have limbs 

 adapted to terrestrial locomotion. The vertebrae are amphicoelous, and most 

 of the ribs have distinct capitula and tubercula. A sternum is present, and also, 

 in many cases at least, an episternum. The quadrate is firmly fixed. Palatine 

 plates are developed comparable to those of Chelonia. There is a parietal 

 foramen. The temporal region is in some covered over by flattened bones as in 

 a Turtle, in others there is a wide lateral temporal fossa bounded above by a 

 superior temporal arch ; in the 

 latter case no quadrato-jugal 

 is developed. An arch corre- 

 Bponding with the zygomatic 

 of Mammals (see Sect. XV. ) is 

 formed by the extension back- 

 wards of the jugal to meet an 

 anterior process of the squa- 

 mosal, both articulating with 

 a downgrowth from the post- 

 frontal. In the pectoral arch, 

 clavicle, coracoid, pro-cora- 

 coid, and scapula are present. 

 The pubes and ischia are closely united, with a common symphysis, as in 

 Mammals ; and the obturator foramen is usually small. The teeth (Fig. 1011) 

 (which are not present in all) are thecodont, and in the higher forms bear a 



FIG. 1011. Left lateral aspect of the skull of Gale- 

 saurus planiceps. or. orbit. (After Nicholson 

 and Lydekker.) 



