64 



THE BIOLOGY OF BIRDS 



entire cells — including the nuclei — of an epidermic thickening 

 changed into horny fibres, running longitudinally, 



are 



transversely, and perpendicularly. In the formation of the 

 ordinary horny covering of the beak, it is only the mantle 

 of the epidermic cells that is changed into the horny fibre, 

 and more or less thin plates result. The egg-tooth and a 

 corresponding rudiment on the tip of the mandible appear 

 earlier than the horny bill, and, as has been stated, their 

 mode of cornification is different. It may be, Rosenstadt 

 suggests, that they represent an old-fashioned armature of 

 the jaws, older than the modern horny bill. 





Fig. 14. — Side view of bird's skull. From a specimen, par., parietal 

 S.O., supra-occipital ; sq., squamosal ; Q., quadrate ; fr., frontal ; as., ali-' 

 sphenoid ; os., orbitosphenoid ; o.F., optic foramen ; me., mesethmoid ; 

 L., lachrymal ; na., nasal ; pmx., premaxilla ; no., nostril ; n.p., nasal pro- 

 cess of premaxilla ; o.p., orbital process of frontal ; j., jugal ; pal., pala- 

 tine ; PT., pterygoid; Q.J., quadrato-jugal ; art., articular of the lower 

 jaw or mandible (mn). 



(c) The complex lower jaw is made up of six bones on 

 each side, thus revealing affinity with reptiles and marked 

 contrast to mammals (where the mandible is simple), but 

 from the present point of view it is more interesting to 

 notice its loose articulation with the movable quadrate, for 

 this increases the gape — an important point when the food 

 has to be swallowed without chewing and without too many 

 questions in regard to size. With the absence of any 

 mastication we may associate the slenderness of the infra- 



