BIRDS. 



337 



All existing birds are toothless, but the dental ridge (p. 1 9) 

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

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



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

 tabulum; /'/, ilium ; is, ischium; p l , postpubis; /, prepubis. 



Hesperornis) or in alveoli (IcJitJiyornis). 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- FIG. 332. Transverse sec- 



, T . . f A v tion through the beak of an 



ach always consists of two divisions, embryo ^ after Rb - se ^ 

 an anterior glandular stomach or dental ridge ; bone, black, 

 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 



