CROCODILIA 257 



walked on all fours, others occasionally or habitually walked 

 upright, after the manner of birds, with which they had many 

 structural features in common. Their size was so great that the 

 footprints measured from 14 to 18 inches. The length was 

 sometimes 60 to 70 feet and the height nearly 20 feet. 



The earliest turtles were found in the Triassic of Europe. 

 True lizards appeared and turtles abounded in the Jurassic. 

 The Pterosau'ria appeared in the Jurassic. They had toothed 

 jaws and were winged like a bat. The spread of wings did not 

 exceed 3 feet. In the Cretaceous Period the spread of wings was 

 20 feet. Gigantic carnivorous marine lizards swarmed on the 

 Atlantic and Gulf coasts and in the interior seas of that time. 



In the tertiary period reptilian life shows a great change, the 

 animals being neither so large nor so varied. 



All the fossil snakes, except one found in the cretaceous, have 

 been found in the tertiary period.^ 



It is plain to be seen why the Mesozoic Era is called the "age 

 of reptiles," and how closely related the reptiles are to both 

 amphibians and birds. They differ from amphibians in having 

 bodies covered with scales, in having but one occipital condyle, 

 in having the embryonic membranes, the amnion and allantois; 

 in Crocodilia, in having a four-chambered heart ; in never having 

 gills, and in never having a tadpole stage. They differ from birds 

 in having scales, but never feathers, and in the circulation (being 

 poikilothermal) . If we include the extinct forms, there will be 

 found many points of similarity between reptiles and birds. 

 Hence it is believed that amphibians, reptiles, and birds have a 

 common ancestry. 



Economic Importance. — As has been mentioned, many rep- 

 tiles are used for food. The skins of rattlesnakes and boas are 

 made into bags, cases, boots, saddle cloth, etc. The oil of the 

 rattlesnake and boas is valuable, and that of the copperhead is 

 used in medicine. The scales of the tortoise-shell turtle are 

 valued for combs and ornaments, and the oil from its eggs is 

 used in dressing leather. From alligators we get valuable 

 leather, oil, and musk. The teeth, flesh, hide, and oil of croco- 

 diles are valued. 



Most of our common forms are not dangerous to man, and, 

 1 Parker and Haswell. 

 17 



