XTTI PHYLUM CHORDATA 313 



CLASS IV. REPTILIA. 



Reptiles, Birds, and Mammals are associated together as having 

 in common certain features in which they differ from lower 

 Vertebrates. The most important of these is the occurrence in all 

 three classes of certain embryonic membranes termed the amnion 

 and the cdlantois t to be described subsequently. The term Amniota 

 is, accordingly, frequently used for the group formed by these 

 three highest classes of the Vertebrata. 



The classes Reptilia and Aves are much more closely allied with 

 one another than either of trfem is with the Mammalia ; and the 

 two first are sometimes associated together under the title of 

 Sauropsida. The following are some of the most salient 

 features of the Sauropsida when compared with the other 

 Vertebrates : 



The integument always gives rise to important and characteristic 

 exoskeletal structures in the form of scales or feathers; the derm is 

 may or may not take part in the formation of an exoskeleton. 

 The skull is well ossified : it rarely in the adult state contains a 

 distinct parasphenoid. There is a single occipital condyle borne 

 on the basioccipital. The basisphenoid is a well-developed bone. 

 The mandible articulates with the skull through the intermediation 

 of a quadrate, and consists of five or six bones on each side. The 

 ankle-joint is an articulation between the proximal and distal 

 divisions of the tarsus. As in the Amphibia, there is a cloaca into 

 which the rectum and the renal and reproductive ducts open. The 

 heart consists of two auricles and a ventricle which is sometimes 

 incompletely, sometimes completely, divided into two parts. 

 Branchiae are never present at any stage. The mesonephri are 

 never the functional renal organs of the adult, but are always 

 replaced by metanephri. Both an amnion and an allantois are 

 present in the embryo, the latter becoming highly vascular and 

 acting as a temporary embryonic organ of respiration. 



The class Reptilia comprises four orders having living repre- 

 sentatives, in addition to a number of extinct groups. In the 

 Mesozoic period the class reached its maximum both in the number 

 of its representatives and the size which many of them attained ; 

 at that period they were very unmistakably the dominant class 

 of the Animal Kingdom. In the Tertiary period they underwent 

 a decline, while the Birds, and, in a yet higher degree, the 

 Mammals, were gaining a preponderance over them. The living 

 Reptiles are the Lizards and Chamaaleons, the Tuataras, the 

 Snakes, Tortoises and Turtles, and the Crocodiles and Alligators. 

 Though horny scales are not by any means present in all the 

 Reptiles, their occurrence as a complete covering is characteristic 

 of the group and almost peculiar to it. When scales are not 



