VERTEBRATA. 107 



3. Eeptilia (Reptiles). — Eespiration aerial, by lungs, and 

 never by gills. Pulmonaiy and systemic circulations con- 

 nected together either within the heart or in its immediate 

 neighbourhood. Heart of the adult three-chanibered in most; 

 rarely four -chambered. Blood cold. Skull united to the 

 vertebral column by one occipital condyle. Exoskeleton in 

 the form of horny scales or bony plates, or both combined. 



4. AvES (Birds). — Eespiration aerial, by lungs, and never 

 by gills. Bronchial tubes opening on the surface of the lungs 

 into air-sacs. A greater or less number of the bones almost 

 always hollow and filled with air. The skull connected with 

 the vertebral column by a single occipital condyle. Heart 

 four-chambered ; the pulmonary and systemic circulations 

 distinct, and the blood warm. Epidermic appendages in the 

 form of feathers. Pectoral limbs in the form of wings. 

 Animal oviparous. 



5. Mammalia (Quadrupeds). — Eespiration aerial, by lungs, 

 and never by gills. The terminations of the air-passages 

 (bronchi) never connected with air-sacs. Heart four-cham- 

 bered ; the pulmonary and systemic circulations distinct; the 

 blood warm. Skull connected with the vertebral column by 

 two articulating surfaces or condyles. Some part or other of 

 the integument provided at some time or other with epidermic 

 appendages in the form of hairs. The young nourished for a 

 shorter or longer time by means of a special fluid — the milk, 

 — secreted by special glands — the mammary glands. Animal 

 viviparous. 



As regards the general distribution in time of the Vertc- 

 hrata, the earliest certain traces of the existence of this sulv 

 kingdom are found in the Upper Silurian rocks. Here are 

 the remains of Ganoid and Plagiostomous fishes ; and we 

 may fairly anticipate that further research will ultimately 

 result in putting back the first appearance of Fishes at any 

 rate to the Lower Silurian. The class of the Amphibians is 

 not known to have come into existence prior to the com- 

 mencement of the Carboniferous period, but it had attained 

 a great development before the close of this epoch. The 

 class of the true Eeptiles is represented by undoubted ex- 

 amples for the first tmie in the Permian deposits. In the 



