602 



CHORD AT A. 



and approach most nearly to the Theromorphs. The maxillaries, 

 palatines, and ptery golds have united in the living species in the 

 middle line, forming a hard palate and forcing the vomers upwards 

 into the nasal region. This same process has carried the choana 

 (fig. 630, Cli) to the 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, abdominal ribs and an ab- 

 dominal sternum. The jaws are extended into a long snout, and 

 the teeth, which occur only on the margins, are placed in sockets 

 (alveoli). The four-chambered heart has already been described 

 (p. 592). The animals move slowly on land, but in the water, 

 thanks to their strong, keeled tail, they are very active. 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. 

 Alligator lucius* alligator ; Crocodilus,* most species Old World, one, C. 

 americanus,* occurring in our southern waters. 



Order IX. Pterodactylia (Pterosauria). 



Extinct reptiles of the Jurassic and cretaceous, adapted for flight. 

 The bones were hollow and the wings were broad membranes, supported, 

 like those of a bat, by the body and the greatly elongated fifth digit of the 



FlG. 631. Dimorpliodon, a pterodactyle. (After Woodward.) 



fore limbs. Some were sparrow-like in size and some, Pteranodon, had a 

 wing expanse of twenty feet. Yet one of these large forms from Kansas 

 had its pelvic opening so small that its eggs could not have been more 

 than half an inch in diameter. 



