258 BRANCH CHORDATA 



any way, will not attack him if unmolested. A few forms of 

 snakes and crocodiles are dangerous to man. 



Important Biologic Facts. — The embryonic membranes, the 

 amnion and the allantois, first appear in this class. Respiration 

 is by lungs. True nostrils appear. The heart is four cham- 

 bered in Crocodilia. 



Classification. — 



" I have considered the birds, 

 And I find their hfe good. 

 And better, the better understood." 



(McDonald.) 



In existing forms, birds are feathered chordates having no 

 teeth, but from paleontology we learn that some of the fossil 

 birds (Figs. 209-212), as the Archceop'teryx (Fig. 212), of the 

 Jurassic Period, had teeth and a long vertebrated tail. 



Birds have several characteristics in common with reptiles, 

 among which are the large eggs, the lack of a complete dia- 

 phragm, the quadrate bone connecting the lower jaw with the 

 skull, and the single occipital condyle. It is believed that in the 

 early stages of their development birds floated in the air by 

 means of a patagium, or wing membrane, and that even "feath- 

 ers were used at first as a means of sailing down" after having 

 crawled up to some height by the use of claws on the "fingers," 

 vestiges of which are yet shown by many birds, such as the 

 turkey, the vulture, certain ostriches, swans, thrushes, and 

 young gallinules, which have claws variously located on the 

 wing tips. 



Covering. — The lower part of the legs is covered with hard, 

 reptile-like scales, or epidermal shields, for protection. The 

 beak antl claws are also horny epidermal structures. 



