360 



ZOOLOGY 



SECT. 



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 maxillae, 

 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 amphiccelous vertebrae. Some of the fossil Crocodiles 

 reached an immense size. 



4. EXTINCT GROUPS OP REPTILES. 



PROREPTILIA. 



The Permian genera Eryops and Cricotus, formerly assigned to the 

 Stegocephala (p. 285), have been proved, on the ground mainly of their verte- 

 bral structure, to be true Reptiles, and are regarded as the most primitive 

 members of the class with strong Amphibian affinities. 



THEROMORPHA. 



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

 which all have limbs adapted to terrestrial locomotion. The vertebrae 

 are amphiccelous, 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 corresponding with the 

 zygoma of Mammals (see Section XV) is formed by the extension backwards 

 of the jugal to meet an anterior process of the squamosal, both articulating 

 with a downgrowth from the post-frontal. In the pectoral arch, clavicle, 

 coracoid, pro-coracoid, 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. 1022) (which are not present in all) 



are thecodont, and in the 

 higher forms bear a consider- 

 able resemblance to those of 

 Mammals in the regularity of 

 their arrangement in sets, often 

 with large canines or tusks. 

 Palatine teeth are sometimes 

 present. One order, the Pla- 

 codontia, have remarkable 



FIG. 1022,-Left lateral aspect of the skull of Gale- broad crushing teeth on both 

 saurus planiceps. or. orbit. (After Nicholson upper and lower jaws and on 

 and Lydekker.) the palate. 



The Theromorpha mainly 



occur in beds between Permian and Triassic age, and have been found in 

 South Africa and North America, as well as Europe and India. Among 

 them have recently been found many transition forms which tend to bridge 

 over the interval between the Reptilia and the Mammalia. 



SAUROPTERYGIA. 



The typical representatives of this order, such as Plesiosaurus (Fig. 1023), 

 were aquatic Reptiles, sometimes of large size (up to 40 feet), though many 



