158 MANUAL OF ANIMAL BIOLOGY 



prehistoric periods when the living representatives included the 

 enormous land-living Dinosaurs, such as Brontosaurus, shown on 

 page 177, the aquatic Icthyosaurs, also of great size, and the 

 Pterosaurs with the fore limbs modified for air flight and showing 

 other structural features which are believed to mark them as a con- 

 necting link with the true birds. (W. f. 79.) 

 Three important orders of living Reptiles are recognized, namely, 



(1) the Testudinata, which includes the Turtles and Tortoises ; 



(2) the Crocodilini, which includes the Alligators and Croco- 

 diles ; (3) the Squamata, represented by Snakes, Chameleons, and 

 Lizards. 



The Turtles and Tortoises possess a comparatively short body 

 which, in all species, is almost completely enclosed in a hard, box- 

 like exoskeletal shell. No other Vertebrates possess such a struc- 

 ture. This group is very largely aquatic in habitat and ranges 

 in size from a few inches in length and weighing a few ounces to 

 four or five feet and weighing some 500 pounds. (W. f. 80.) 



The Crocodiles are large aquatic Reptiles with an elongated 

 body ending anteriorly in a long snout with a pair of nostrils at 

 the tip. The tail is long and very muscular. The skin is un- 

 usually heavy and almost completely covered with horny exoskele- 

 tal plates. The limbs are well-developed and more or less web 

 may be present between the digits of the hind limbs, but they 

 are not well-adapted for locomotion on land. The eyes protrude 

 above the dorsal surface so that they lie above the surface of the 

 water when the animal is floating. The Crocodiles and Alligators 

 belong to the same family, and show few differentiating features. 

 Thus in the Alligator the snout is broader and less pointed, the 

 body heavier, and their disposition more favorable to strangers ! 



The Squamata are almost entirely terrestrial animals, and the 

 limbs, when present, are well-adapted for locomotion on land. The 

 Chameleons and the Lizards, with few exceptions, possess two 

 pairs of pentadactyl limbs. In the Chameleons, the digits are 

 grouped to permit grasping of objects, such as tree branches. In 

 the Snakes, no limbs are present, but inasmuch as internal vestigial 

 remains of limbs and limb girdles are to be found in various species, 

 it is concluded that they have descended from limbed types. The 

 body is enclosed in a scaly, exoskeletal covering which is shed 

 periodically, and then formed anew as a secretion by the under- 

 lying dermal cells. (W. fs. 81-83, 230.) 



