ADAPTATIONS OF THE SKELETON 



63 



notice the fusion of skull bones in the extinct flying Ptero- 

 dactyls, which do not seem to be related to birds. 



(b) Very characteristic is the exaggeration of the pre- 

 maxillae which form the bulk of the beak, and the absence 

 of teeth in modern birds is compensated for by the very 

 effective horny bill. 



S.O 

 'S.O 



.-EX.0 



PTy 



Fig. 13. — Dorsal and ventral view of a bird's skull. From a specimen. 

 S.O., supra-occipital ; pa., parietal ; fr., frontal ; na., nasal ; pm.x., pre- 

 maxilla ; l., lachrymal ; Q.J., quadra to-jugal ; m., mandible 01 lower jaw. 

 c, occipital condyle, formed from the basi-occipital ; ex.o., ex- 

 occipital ; E., ear-hole; Q., quadrate ; SQ., squamosal ; B.T.,basi-temporal; 

 PTY., pterygoid ; mx., small maxilla ; v., vomer ; j., jugal ; bs., basi- 

 sphenoid. 



The alleged hints of a dental system in birds are probably 

 altogether mistaken. " Tooth-papillae " have been described 

 in embryo parrots, but they are mere analogues of teeth, 

 with no relation to true dental primordia. The dental 

 ridges of Rose are connected with the development of the 

 horny bill. According to Hide (19 12), who has gone care- 

 fully into the question, there is not in birds any trace of a 

 dental ridge or of dentine. 



The minute structure of the egg-tooth has been care- 

 fully studied by B. Rosenstadt (1912), who finds that the 



