Age of Reptiles, when the giant Dinosaurians, some of which 

 measured over a hundred feet in length, roamed the swamps, 

 and the whale-like Ichthyosaurians swam in the seas. 



In reptiles, as in amphibians, the chief variations in the body 

 skeleton are correlated with differences in external body 

 form. In the short compact body of the turtles and tortoises 

 the number of vertebrae is much smaller than in snakes. 

 Some turtles have only 34 vertebrae; certain snakes as many as 

 400. The reptilian skull, in the number and disposition of 

 its parts and in the manner of its attachment to the spinal 

 column, resembles that of the birds, although the cranial bones 

 remain separate, not fusing as in birds. All of the reptiles, 

 except the turtles, are provided with small teeth which serve, 

 generally, for seizing or holding prey and not for mastication. 



Reptiles breathe solely by lungs, of which there is a pair. 

 They are simple and sac-like, the left lung being often much 

 smaller than the other. In turtles and crocodiles the lungs 

 are divided internally by septa into a number of chambers. 

 Because of the rigidity of the carapace, or "box", of turtles the 

 air cannot be taken in the ordinary way by the use of the 

 ribs and rib muscles, but has to be swallowed. The reptilian 

 heart consists of two distinct auricles and of two ventricles, 

 which in most reptiles are only incompletely divided, the 

 division into right and left ventricles being complete only 

 among the crocodiles and alligators, the most highly organized 

 of living reptiles. 



The organs of the nervous system reach a considerable de- 

 gree of development in the animals of this class. The brain 

 in size and complexity is plainly superior to the amphibian 

 brain and resembles quite closely that of birds. Of the organs 

 of special sense those of touch are limited to special papillae 

 in the skin of certain snakes and many lizards. Taste seems 

 to be little developed, but olfactory organs of considerable 

 complexity are present in most forms, and consist of a pair 

 of nostrils with olfactory folds on their inner surfaces. The 

 ears vary much in degree of organization, crocodiles and 

 alligators being the only reptiles with a well-defined outer 

 ear. This consists of a dermal flap covering a tympanum. 



