70 STORY OF THE REPTILES 



f 

 been mixed all over the body on at least one creature, 

 as we find them now mixed on the legs of birds. Then 

 also a little lower, perhaps, the mammals and reptiles 

 could not have been distinguished from ea-ch other by 

 their covering — or, indeed, by anything else ; for all 

 classes were very much merged into each other at an 

 early date. Even now the pangolins, the armadillos, 

 and other mammals show scales and plates ; so that 

 some reference to internal anatomy is necessary in 

 certain cases to distinguish reptiles and mammals. 



Many distinguishing features might be mentioned, 

 but the presence of glands for nourishing the young 

 by milk is peculiar to no class but mammals. It gives 

 them their name, thus separating them from all others.' 

 Outwardly, then, a reptile may be defined as a strictly 

 lung-breathing, cold-blooded vertebrate usually cov- 

 ered with scales or horny plates, while the young are 

 hatched from large eggs and are never nourished by 

 means of milk-glands, and never have a tadpole state. 



Besides the complete abandonment of gill-breath- 

 ing there is found now with the reptiles the first 

 sternum or breast-bone having the ribs completely 

 reaching it. As noted, there is some evidence that 

 amphibians once had ribs nearly complete, but have 

 lost them. The fishes hinted at the breast-bone, but 

 it was useless; the amphibians had it to swing the 

 fore limbs to but not to join the ribs to, but in the 

 reptiles it first becomes an implement of respiration, 

 whereby the lungs are made to open and shut. In 

 tortoiselike reptiles it is absent, and the ribs are stifE- 

 ened into the shell, but a muscle called the diaphragm 



