ELEMENTS OF MAMMALIAN ANATOMY 



mian, they doubtless began to differentiate from the 

 stegocephalian amphibians much earlier. Their 

 development was so rapid in the Mesozoic times 

 that they dominated the earth, the sea and the sky. 

 They reached enormous proportions. One of the 

 terrestrial forms, a dinosaur, measured over 90 feet 

 in length; with head erect could reach to a height 

 of 30 feet, although its normal height was but 15 

 feet. The sea forms, or ichthyosaures, were also of 

 large size. They were fish-like in shape and were 

 frequently provided with long slender jaws armored 

 with sharp conical teeth. There were also flying 

 reptiles, Pterosaures, with a wing-spread of over 20 

 feet. Of the twenty-five or more Mesozoic orders, 

 but a few of the more insignificant types remain 

 to-day; such as snakes, turtles, lizards, crocodiles 

 and alligators. 



Aves (Birds). — ''Warm-blooded" animals with 

 feathers: An offshoot of the reptiles. 



8. The Mammalian Stage 



, Mammalia. — Vertebrates suckling their young; 

 hairs never entirely absent; diaphragm complete; 

 heart four-chambered; aorta curved to the left; red 

 blood-cells, when fully formed, without a nucleus 

 and round, except in the camel where they are 

 elliptical. The advent of mammals occurs during 

 the triassic. They are probably related to the 

 reptiles through the theromorphs. The mammals 

 of the early Cenozoic were small and insignificant. 

 However, they soon become the dominant type of 

 animal, so that the Cenozoic becomes known as the 

 age of mammals. The secret of their success is 

 their four-chambered heart and their protective 

 covering of hair. 



