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 l:)ack, ab- 

 dominal ribs (gaslralij), and an abdominal sternum. The Jaws are 

 extended into a long snout, and the teeth, wliich occur only on the margins, 

 are placed in sockets or ali'foli (llifcodoiil). The four-chambered heart 

 has alread)- been described (p. 5^4). The animals move slowl}- on land, 

 but are very active in the water. They ha\e a strong smell, owing to 

 musk glantls in the cloaca and on the untler jaw. The group appeared 

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

 Parasuchia, are extinct. 



Sub Order EUSUCHI.\. Externa! nostrils united, choana posterior; 

 five toes in front, four behind. Gavialis, India, snout lont; and slender. -4///- 

 gator liiciiis* alligator; Crocodiliis.''' most species Old World, one, C. amcri- 

 caiuis/-' 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, bv the body and 



Fig. 5S2. — Pimorpluhloii, a pU-rodaclyle (after Wooihvard). 



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

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

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



Class II. Aves. 



While structurally the birds are near the reptiles, yet liy 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 co\-ered 

 with horny scales and shields; on the toes and occasioiuillv on the fintTers 



