xra PHYLUM CHORDATA 303 



are a very specialised group : their development indicates their 

 derivation from branchiate tailed forms, but there is no palaeonto- 

 logical evidence on this point. 



CLASS IV.-REPTILIA. 



Reptiles, Birds, and Mammals are associated together as having 

 in common certain features in which they differ from lower Verte- 

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

 three classes of certain embryonic membranes termed the amnion 

 and the allantois, 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 them is with the Mammalia ; and the 

 first two 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 dermis 

 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 basi- occipital. The basi-sphenoid 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, 

 BranchiaB are never present at any stage. The mesonephros is 

 never the functional renal organ of the adult, but is always 

 replaced by a metanephros. 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 Chamseleons, 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 occur- 



