546 TEXTBOOK OF ZOOLOGY 



"18. The eggs are meroblastic. 



' ' 19. Fertilization is internal, 'and is effected, with the single ex- 

 ception of Sphenodon, by means of male copulatory organs. 



"20. An amnion and an allantois are formed during development. 



"Numbers 1, 2, 6, 7, 8, 14, 16, 18, and 20 separate the Reptiles 

 from the Amphibia. Numbers 9 (b), 10, 12, and 13 separate them 

 from the Birds and Mammals. Numbers 3, 8, and 11 separate them 

 from the Mammals." 



The majority of living forms are covered by scales. The turtles 

 have epidermal scutes as an external covering ; and the crocodiles 

 have a tough skin w^ith osseous plates in it. Many living reptiles 

 are capable of very rapid movement ; and the Pterosauria, an ex- 

 tinct order, had wings. Only one lizard and a relatively small 

 number of snakes are poisonous. It is thought that the poison gland 

 is a recent development among reptiles. 



At present herpetologists place all known reptiles into nineteen 

 groups or orders. Only four of these orders have living representa- 

 tives. This indicates that Reptilia is an old class of vertebrates 

 which is now decadent. 



FOSSIL REPTILES 



A study of the fossils of reptiles reveals that during the Triassic, 

 Jurassic, and Early Cretaceous geological periods reptiles were the 

 dominant animals on earth. They occupied and dominated all types 

 of habitats then much as mammals do now. Many of these ancient 

 reptiles were no larger than the common lizards of today, but others 

 were the largest vertebrates that have ever lived. Brontosaurus, 

 "the thunder lizard," was nearly seventy feet long and weighed 

 approximately forty tons. This enormous reptile lived in the shallow 

 waters of ponds and bays and fed on the plants that grew in the 

 mud along the margins of the water. Tyrannosaurus, "the king 

 lizard," reached a length of forty-seven feet and a weight greater 

 than that of any elephant. Since it was entirely carnivorous in its 

 feeding habits, it is easily the most terrible animal that has ever 

 lived on earth. Many variations in form and size have bridged the 

 gap between the largest and the smallest. The five or six known 

 families of flying reptiles varied in expanse of wings from a few 

 feet to as much as twenty-five feet. As a group, they dominated 

 the air for several million years. A few small wormlike burrowing 



