66 



The second main branch of the Sauropsida leads upwards 

 to the extinct Ornithoscelida (Deinosauria), the Crocodilia 

 being given off as a side branch in the direction of the 

 other Reptilia^ and rising to a rather higher level of organisa- 

 tion. 



The Aves are derived from the Ornithoscelida, and are 

 mainly distinguished from other Sauropsida by the modifica- 

 tion of the epidermal exoskeleton into feathers. The 

 extinct groups belonging to this region shew a most 

 interesting series of transition forms between Reptiles and 

 Birds, and most of the Ornithoscelida lead gradually up 

 through various extinct forms known as fossils to the Birds 

 of the present day. 



The Saururae {Aixhceopteryx) were more primitive than 

 any existing forms, and possessed a long tail, to which how- 

 ever feathers were attached. From this group the remaining 

 Aves, in which the tail is short and formed of coalesced 

 vertebrae, have descended in two series : the Ratitae, in which 

 the sternum is retained in its primitive flat condition, and the 

 Carinatae, in which it is provided with a more or less well 

 developed keel. The Aves have become differentiated into a 

 large number of groups which, however, show very little 

 structural modification. They attain a higher level of 

 organisation than any of the other Sauropsida. 



The first ancestral Mammalia probably arose from the 

 Proto-Amniota, at the base of the Sauropsida. Caldwell's* 

 recent investigations show that the most primitive Mammals 

 now existing differ from the Amphibia, and agree with Rep- 

 tiles and Aves in having a large amount of food-yolk in the 

 ovum. Very possibly, this character was acquired by the 

 Proto-Amniota, and inherited by the two lines of descent 

 (Sauropsida and Mammalia) into which that group divided. 



* See Nature, vol. xxx, p. 577. 1884. 



