The Classification of Animals. 205 



a fish-like condition in the extinct Ichthyosaurus, etc. They 

 never, however, possess gills. The epidermis possesses a 

 skeleton in the form of scales. The skull articulates with 

 the vertebral column generally by one condyle only ; it is almost 

 completely ossified. The gut terminates in a cloaca; the 

 heart is at least three-chambered, and the ventricle may be 

 incompletely or completely divided into two chambers. 

 There are, as in Aves and Mammalia, twelve pairs of cranial 

 nerves. The embryo, like that of the higher Vertebrata, has 

 an amnion. The Reptiles are divisible into nine orders, of 

 which five, viz. the Ichthyosauria, Plesiosauria, Theromorpha, 

 Dinosauria, and Pterosauria, are totally extinct. The orders 

 with living representatives are the Testudinata, Lepidosauria, 

 Rhynchocephalia, and Crocodilia. We will briefly give the 

 characters of the extinct orders first. 



1. Ichthyosauria. — These are the well-known " fish-lizards " 

 of the middle period of the earth's history. Large, often very 

 large, creatures, with fish-like paddles, and a tail formed by an 

 expansion of the integument, like the tail of a whale. 



2. Plesiosauria^ — The long-necked Plesiosaurus is familiar 

 to everybody. It had paddles, in which the individual bones 

 are less numerous than in the Ichthyosattrus. They are also 

 mesozoic in time range. 



3. Theromorpha. — These reptiles are interesting on account 

 of the many points of resemblance which they show to the 

 Mammalia. Their teeth, for example, often show much speciali- 

 zation — canines, molars, etc., being distinguishable ; whereas in 

 reptiles generally the teeth are numerous and all similar. 



4. Dinosatma.-'—Ths last group of extinct reptiles present 

 certain likenesses to the Mammalia. The present group is, in 

 many respects, nearly akin to the group of birds. The dino- 

 saurs are mesozoic and were of varied size; some were as 

 small as a crow, others reached a length of sixty feet or so. 

 Their likeness to birds is mainly in the pelvis, but it seems 

 probable, from certain osteological details, that the bones were 

 permeated with air-spaces., 



5. Pterosauria. — In some ways the pterodactyls also re- 

 semble birds. They were flying reptiles with extensive wings. 



