532 



CHORDATA 



back of the skull. Some of the ribs have two heads; the ears and nostrils 

 are provided with valves. A sternum is present and, farther back, ab- 

 dominal ribs (gastralia), and an abdominal sternum. The jaws are 

 extruded into a long snout, and the teeth, which occur only on the margins, 

 are placed in sockets or alveoli (thecodont}. The four-chambered heart 

 has already been described (p. 524). The animals move slowly on land, 

 but are very active in the water. They have a strong smell, owing to 

 musk glands in the cloaca and on the under jaw. The group appeared" 

 in the trias, and of the three sub orders two, the Pseudosuchia and 

 Parasuchia, are extinct. 



Sub Order EUSUCHIA. External nostrils united, choana posterior; 

 five toes in front, four behind. Gavialis, India, snout long and slender. Alli- 

 gator lucins* alligator; Crocodihts* most species Old World, one, C. ameri- 

 'i-(inns,* occurring in our southern waters. 



Order IX. Pterodactylia (Pterosauria) . 



Extinct Mesozoic reptiles, adapted for flight. The bones were hollow, the 

 wings were broad membranes, supported, like those of a bat, by the body and 



FIG. 582. Dimorphodon, a pterodactyle (after Woodward). 



the very long fifth finger. Some were sparrow-like in size and some, Pteranodon, 

 had a wing expanse of twenty feet. One of these large forms, had its pelvis 

 so small that its eggs could not have been more than half an inch in diameter. 



Class II. Aves. 



While structurally the birds are near the reptiles, yet by the develop- 

 ment of wings and the feathering of the body the group is strictly circum- 

 scribed. The skin in some places, as the lower part of the legs, is covered 

 with horny scales and shields; on the toes and occasionally on the fingers 



