//. TYPES IN DETML IVITH THEIR SUBORDINATE GROUPS. 43 



Group 2. HYDROSAURIA . . . Cloacal opening oval in shape, its 



longer axis longitudinal. A single 

 organ of copulation in the male, 

 anterior to the cloaca. Scales 

 large and irregular, often reinforced 

 by bony plates, which may coalesce 

 to form dorsal and ventral shields. 

 Mainly aquatic in habits. Quad- 

 ratum immovably attached to 

 skull. 



Order 3. Chelonia Body enclosed by dorsal and ven- 

 tral shields, formed partly by ele- 

 ments of the endo-skeleton and 

 partly from the integument. Teeth 

 functionally replaced by homy 

 beak, with sharp cutting edge. 



X Chrysemys, Chelone. 



Order 4. Crocodilia Body elongated, covered by large 



plates, which do not coalesce. 

 Thoracic and abdominal sterna 

 present, connected by ventral and 

 dorsal ribs. Teeth large, in alveoli. 

 X Alligator, Crocodilus. 



Our knowledge of Reptilia is greatly increased by the discovery of 

 several important fossil Orders. The Theromorpha, very primitive in 

 some particulars, possessed certain anatomical characteristics which are 

 suggestive of the Mammalia, and are thus considered by many as ances- 

 tors of the latter group. The Mosasauria, an Order of snake-like 

 marine reptiles, were allied to both Lacertilia and O^hidia, and it is pos- 

 sible that surviving members of this group are occasionally seen and 

 called " sea-serpents." 



The Pterosauria were allied to the Lacertilia, and possessed mem- 

 branous expansions of the integument of the arms and fingers, by which 

 they could fly. The Pleisiosauria and Ichthyosauria were hydro- 

 saurs, the former somewhat resembling turtles, the latter, crocodiles. 

 The Dinosauria included some enormous terrestrial forms with mas- 

 sive skeletons, e.g., Brontosaurus and Iguanodon. Other smaller Dino- 

 saurs may have been the precursors of birds. They walked mainly upon 

 their hind feet, possessed pneumatic cavities in their bones, and showed 

 many other avian characteristics. An important form is Compsognatkus, 

 which shows affinity to Archceopteryx (see introduction to Aves). 



