300 PRACTICAL LESSONS IN SCIENCE. 



covered with scales ; the pectoral and pelvic arches are present, 

 and the bones of the head and jaws are more fully consolidated 

 than in the snakes. This group includes the tree swifts, horned 

 toads, glass snakes, which have hidden rudimentary limbs, lizards, 

 skinks, etc. Lizards are more abundant in the warmer parts of 

 the world, where they are common everywhere about walls, build- 

 ings, rocks, trees, etc. 



The CrocodWaare not only larger than other reptiles, but they 

 seem in every way more highly developed. The alligators of our 

 Southern States are the only representatives of the class in this 

 country. 



Lizards and crocodilian reptiles existed in great numbers and 

 variety and attained to an enormous size during middle geologic 

 times, so that one period is called the Age of Reptiles. 



The Chelonia, are a well-marked group of animals. They have 

 a short stout body, with the dorsal and ventral surface covered 

 with osseous shields, which in turn are covered by horny plates 

 or a leathery skin. The neck and tail are flexible but the dorsal 

 and sacral vertebra? help to form the carapace, uniting with ribs 

 and overlying bony plates. The plastron or ventral plate seems 

 to consist of membrane bones. There are no teeth, but the jaws 

 are incased in horny sheaths which usually have sharp cutting 

 edges. The eye is furnished with eyelids, and respiration is ef- 

 fected by means of air that is swallowed . This group includes 

 the land tortoise, box turtles, terrapins, snapping turtles, soft- 

 shelled turtles, etc. 



The Aves or Birds. The birds are warm-blooded oviparous 

 animals, clothed with feathers. They have four extremities, two 

 adapted for walking or swimming and two fitted for service as 

 organs of flight. 



The bones of birds are more compact and lighter than those of 

 reptiles. The cranial bones are fused together into a light, firm 

 skull, which is articulated to the vertebral column by a single 

 condyle. The face bones unite to form projecting beaks, the mar- 

 gins of which are covered with a horny substance that supplies 

 the place of teeth. The vertebral column has a long, flexible, 



