604 The Animal Kingdom 



Order 4. Saurischia. Examples: extinct "reptilelike" dinosaurs, carnivor- 

 ous and large amphibious forms. 



Order 5. Ornithischia. Example: extinct "birdlike" dinosaurs, herbivorous, 

 many armored and horned. 



Subclass 6. Synapsida. Extinct forms, ancestral to mammals. 



Order 1. Pelycosauria. Examples: extinct mammal-like reptiles, which 

 lived during the Permian, similar to stem reptiles. 



Order 2. Therapsida. Example : extinct mammal-like forms which lived dur- 

 ing the late Permian and Triassic. 



Of the throngs of reptiles that crowded the Mesozoic landscape, 

 only a few species survive in our modern world. Even the largest of 

 the present-day forms are insignificant when compared to such giants 

 as the 80-foot Brontosaurus of the Jurassic. Of the many and varied 

 reptiles that dwelt in ages past, only four groups survive : the turtles, 

 the lizards and snakes, the crocodiles and alligators, and Sphenodon. 

 As these groups diverged far back in the geologic past, they seem but 

 little related to each other. Each has its distinctive structure, appear- 

 ance, and habits. 



The Order Chelonia (the Turtles). — The deliberate, slow-moving 

 turtle has little to fear from "his enemies for w4ien approached he simply 

 withdraws within his shell and becomes an immobile, bone-enclosed 

 animal. Speed has thus been sacrificed for the sure protection of the 

 heavy shell. The shell is formed of the upper arched carapace and the 

 lower flattened plastron; these are joined between the two pairs of legs 

 by the bridge. A number of bones joined by irregular sutures form the 

 shell. These bones are covered by bony epidermal lamellae. Such 

 protection as afforded by this heavy shell has necessitated many other 

 structural changes in the fundamental reptilian form. First, the ribs 

 and vertebrae are immovably incorporated into the carapace ; and, 

 second, the girdles shift their position to one under the ribs. This 

 first change has resulted in the degeneration of many trunk muscles 

 and a corresponding development of leg and girdle muscles. The latter 

 shift of the girdles is correlated with a peculiar leg development in 

 which they are moved far apart and crooked at the elbows and knees. 

 Thus when moving, the elbows of the forelegs are swung forward, 

 parallel with the neck. The awkward body moves by pulls and pushes 

 and weaves and twists as the animal progresses forward. 



The rigid shell has also necessitated certain important changes 

 in the mode of respiration of the turtles. Air is inspired and expired 



