CLASS AVES. 



Sub-Classes. Orders. 



1. Archaeoraithes. 1. Saiirurae. 



2. Neornithes. 



1. Ratitse 



2. Carinatje 



Sub-Orders. 



1. Odontolcae. 



2. ^Epyornithes. 



3. Apteryges. 



4. Immanes. 



5. Megistanes. 

 G. Rhese. 



7. Struthiones. 



( 1. Odontormae. 



2. Impennes. 



3. Pygopodes. 

 . 4. Tubinares. 



&c. &c. 



The earliest known birds exhibit a distinct approach to 

 the Reptilia in several characters, but do not afford any indi- 

 cation as to the particular group from which they evolved. 

 The general opinion is, that they are more closely related to 

 certain Dinosauria than to any other forms hitherto discovered. 

 The facts at present known merely show, that birds had 

 already become differentiated and acquired the characteristic 

 covering of feathers before the tail atrophied and the pelvis 

 fully developed ; that the vertebrae next began to acquire their 

 saddle-shaped articulations ; and that after this the teeth were 

 habitually lost, at least in the adult. There is also reason 

 to believe that the Ratite and Carinate birds were already 

 differentiated before the vertebrae and beak had assumed the 

 characters now universal in the Class. This evolution, however, 

 was almost, if not quite completed during the latter part of 

 the Cretaceous period ; and the known Mesozoic birds are 



