2 THE STRUCTURE OF THE FOWL 



(1) The Archceorniihes (primitive birds) con- 

 tain only one known member, the fossil 

 Archaeopteryx. (2) The Neornithes (modern 

 birds) are distinguished by shortness of tail 

 and fusion of the metacarpal bones of the palm, 

 and may be divided as follows : — 



(a) Ratitae. Running birds with rudiment- 



ary wings and a flat breast-bone, e.g. 

 the ostriches. 



(b) Odontolcse. Extinct fossil birds with 



numerous teeth carried in long jaws, 

 e.g. Hesperornis. 



(c) Carinatse. Flying birds with well- 



developed wings and a breast-bone 

 provided with a keel to which the 

 powerful muscles of flight are attached. 

 It is in this division that the domestic 

 fowl is included. 

 Structurally, birds share with mammals the 

 distinction of being the most highly specialised 

 of vertebrates. Many of their modifications j 

 are adaptations for flight. Tlie fore-limb is \ 

 changed into a wing on which are large feathers I 

 capable of offering a large elastic surface to ll 

 the resistance of the air. A wing-membrane 

 stretching between the arm and the forearm 

 increases the area of this surface. In order 



