372 



ZOOLOGY 



SECT. 



considerable resemblance to those of Mammals in the regularity of tlieir 

 arrangement in sets, often witli large raiiiiie.s or tusks. Palatine teetli are 

 sometimes present. One or.ler, tlie Placodontia, liave remarkable Ijroad crusliing 

 teeth on both upper and lower jaws and on the palate. 



The TheromoiT)ha only occur in beds of Permian and Triassic age, and liave 

 l)een found in .South Africa and North America, as well as Europe and India. 



Among them have recently been found 

 J many transition forms wliicli tend to 



s l)ridge over the interval Ijetween the 



S i\02)tilia and the Mammalia. 



-M 

 "a 



p Sadroptkkvcia. 



s 



^ The typical representatives of this 



•z order, such as Plesiosaurus (Fig. 1012), 



J were aquatic Reptiles, sometimes of 



^ large size (up to 40 feet), though many 



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Fii;. 1013.— Plesiosaurus, i.eetciiil arch. cor. 

 coracuid ; e. epistenium ; ;//, glenoid cavity ; 

 xr. scapula. (After Zittel.) 



\\eie quite small. They had a lizartl- 

 like body, a very long neck, sup- 

 porting a relatively small head, and a 

 short tail wliich supported a vertical 

 caudal tin ; the lindjs were modified to 

 form swimming-paddles. In older and 

 less specialised members of the group, 

 liowever, tiie limbs were not ])addledike, 

 but adapted for walking. 



The spuud column of the Sauio- 

 ])terygia is characterised by the great 

 length of the cervical, and the shortness 

 of the caudal region. The vertebrae are 

 usually amphiceeloiis. The sacrum consists of eitlier one or two vertebra-. There 

 is no sternum. In the skull there are large i)remaxilla> ; a bony palate is 

 a1)sent ; a transverse Ijone is ])rcsent. One temjjoral arch only is dcveh)])ed. 

 'J'iieie is a well-marked parietal foramen. The ring of bony plates (developed in 

 the sclerotic) found in the orbit of some fossil Reptiles is not developetl. The 

 quadrate is not movable. Tlie pectoral arch (Fig. 1013) presents some remark- 



