REPTILIA. 39 



CLASS REPTILIA. 



This class is alone among the vertebrates in comprising a 

 greater number of extinct than of living orders. The Mesozoic 

 era is well named the Reptilian Age, for reptiles then had a 

 most wonderful development in variety, number and size. They 

 were not only the largest creatures of the time, but some of 

 them were undoubtedly the largest land-animals that ever lived. 

 The class began in the Permian, culminated in the Jurassic and 

 Cretaceous, with generalized forms, combining features possessed 

 by birds and mammals, and has now become much degraded in 

 size and numbers, although perhaps more highly specialized. 



It is impossible to briefly deflne even the living reptiles so as 

 to exclude the amphibians upon the one hand, and the birds 

 and mammals upon the other. Their affinity with birds is so 

 close that the two classes are grouped together as the Sauropsida 

 (see page 35). Notwithstanding their external dissimilarity they 

 are linked by many characters, among which are the following : 

 Oviparous, or ovoviviparous ; eggs large and similar; aerial res- 

 piration ; nucleated red blood-corpuscles ; compound rami ; jaw 

 articulated to the skull by quadrate bone; single occipital con- 

 dyle. 



Some of the characters which separate reptiles from birds are 

 as follows : — blood cold ; skin usually bearing scales or plates, 

 never producing feathers, heart with usually imperfectly divided 

 ventricle, but always some mingling of venous and arterial blood ; 

 lungs not connected with air sacs ; thorax and abdomen not dis- 

 tinguished ; limbs sometimes M'anting, and anterior pair never as 

 wings. 



For some reason unknown the orders characteristic of the 

 Mesozoic disappeared with the Cretaceous, and those of the 

 Eocene were mostly remnants of the lesser types. But there 

 has been yet a decadence in the existing orders, in America, 

 since the Miocene. 



The Ophidia are the latest type, and mostf characteristic of the 

 Tertiary and Quaternary. None have been found in America 



