BIRDS. 



343 



or flipper-like. As the yolk is absorbed, the yolk sac is drawn 

 into the body cavity, and the abdominal walls close. Then the 

 shell is broken, in most birds by means of a calcareous or 

 horny growth at the tip of the upper jaw (egg-tooth), and the 

 young begins its free life. 



As far as is at present known, birds appeared (Archceoptcrv.v, 

 Laoptcryx) in the Jurassic. In the cretaceous the genera IcJithy- 

 ornis and Hesperornis are found, while in the tertiary the forms 

 are more numerous, although at all times fossils belonging to 

 the group are rare. 



The order of birds is so uniform in its structural features 

 that it is difficult to find important characters to differentiate 

 the twelve thousand known species into convenient groups. As 

 a result, ornithologists have raised a number of minor groups 

 into so-called orders, which are scarcely of family rank, if we 

 are to accept the rules in vogue in other groups of vertebrates. 

 The group by most authors is sub-divided into Ratitae and 

 Carinatae, divisions based upon the presence or absence of a 

 keel to the sternum ; but these divisions are artificial, and do not 

 indicate the phylogeny of 

 the forms concerned. 



ORDER I. SAURUR^E 

 (ARCH^EORNITHES). 



Extinct birds with 

 elongate tail consisting 

 of many vertebrae ; up- 

 per jaw with teeth (low- 

 er unknown) ; vertebrae 

 amphicoelous ; feathers 

 of the normal type, those 

 of the tail in pairs, a 

 pair to each vertebra. 

 Only two specimens are 

 known, both coming 

 from the lower Jurassic 

 slates of Bavaria. These belong to the genus Arch&opteryx, 

 but may represent two distinct species. One is about the size 



FIG. 336. Restoration of Archicopteryx, 

 after Pycraft. 



