4O Verlebrata. 



brates in that there are only single bones in these 

 regions, the separate bones, radius and ulna, which 

 are present in these parts of other vertebrates being 

 here united. The frogs, toads, pipas, and tree frogs 

 are the most striking examples of this order. 



CHAPTER VIII. 



CLASS 3, REPTILES. 



23. Characters of Reptilia. Tortoises, lizards, 

 snakes, and crocodiles are the leading forms included 

 in this large third class of vertebrate animals, a class 

 often confounded with the amphibians, but differing 

 therefrom in many striking and characteristic respects. 

 Reptiles are invariably provided with a very distinct 

 epidermic clothing of scales which differ essentially 

 from the dermal scales of the foregoing groups. The 

 scales of reptiles being epidermal, and not parts of the 

 derm is or true skin, are often shed and replaced, as 

 in snakes, and they are sometimes hard and thick, as 

 in the tortoise shell of commerce and in the mail 

 clothing of the crocodile. This firm covering may be 

 supplemented by a dermal bony layer, as in croco- 

 diles or tortoises, but these indurations of the dermis 

 are never superficial. The blood is cold ; the aortic 

 arches never bear gills, nor is there ever branchial 

 respiration in any stage of existence among the animals 

 of this group. The heart consists of three cavities, 

 two auricles and one ventricle j but the latter is often 



