BIRDS. 



337 



All existing birds are toothless, but the dental ridge (p. 19) 

 is formed in the embryo of at least a few forms. In several 

 fossil birds teeth were present, either in grooves (Archaopteryx, 



FIG. 331. Pelvis of Apteryx, after Marsh, from Wiedersheim. a, ace- 

 tabulum; /7, ilium ; is, ischium; /*, postpubis; /, prepubis. 



Hesperornis) or in alveoli (Jchthyornis). Many modern birds 

 have the horny sheath of the beak produced into horny tooth-like 

 processes, which in many cases are supported by corresponding 

 elevations of the bone. The tongue 

 is well developed and protrusible, and 

 exhibits many modifications in form. 

 In most birds the oesophagus is of 

 the same size throughout, but in 

 grain-eating birds and birds of prey it 

 has an enlargement or crop which 

 serves as a reservoir of food, and in 

 many cases is glandular, and hence 

 plays a part in digestion. The stom- 

 ach always consists of two divisions, 

 an anterior glandular stomach or 

 proventriculus and a second muscu- 

 lar stomach or gizzard, the muscles of the latter, which radiate 

 from a tendinous centre on either side, being best devel- 

 oped in the grain-eating forms. A < pyloric stomach ' occurs 



FIG. 332. Transverse sec- 

 tion through the beak of an 

 embryo tern, after Rose, d, 

 dental ridge ; bone, black. 



