266 AN INTRODUCTION TO ZOOLOGY. 



ing to the conditions under which it is placed. The 

 higher forms of the Urodela, instead o£ having fish- 

 like biconcave vertebrae like the lower forms, have 

 vertebra which are opisthocoelous, i.e. hollow on the 

 posterior surface only. The Anura, on the contrary, 

 have vertebrae which are procoelous, i.e. hollow in 

 front only. All these varieties of vertebra are also 

 represented among the reptiles. 



Eeptiles. 



liiuks with other Classes. — The question of the 

 relationship of Reptiles with the lower vertebrates 

 would be a very difiScult matter to discuss ; but the 

 existence of extinct reptiles with fish-like biconcave 

 vertebrae afibrds at least one link (fig. 99). There 

 is more to be said about the relationship of reptiles 

 with higher groups ; and, as will be stated under the 

 head of these classes, we have very distinct evidence 

 of the relationship of reptiles with birds, and also of 

 the relationship of reptiles with mammalia. 



Characters of Eeptiles. — The Eeptiles never 

 have a larval stage like that of Amphibia, but are 

 always air-breathing animals. They are slow in 

 movement, and are cold-blooded. The bones of the 

 pectoral girdle may be again named here, for their 

 arrangement in reptiles presents a type fi'om which 

 may be derived the arrangements of them seen in the 

 remaining classes of the vertebrates. Birds and Mam- 

 mals. The pectoral girdle consists on each side of a 

 dorsal part, the shoulder-bone, or scapula, articulating 



