No.' 633] INHERITANCE IN THE OSTRICH 291 



sists of a single, smooth or papillose thickened area of 

 the skin, resting upon a bony support; but in the ostrich, 

 as in other birds and in reptiles, it is constituted of a 

 number of separate and distinct thickenings, somewhat 

 regular in their arrangement, which give the appearance 

 of a rounded or- angular mosaic or tessellation (Figs. 5 

 and 6). This is typically shown on the under surface of 

 the toes of birds and lizards, where the elements tend to 



be elongated and present a coarsely villous effect. Where 

 the skin is scaly each callous constituent corresponds 

 with an individual scale, but the latter has evidently no 

 determining influence upon the form assumed, for the 

 same tessellated arrangement is found over the sternal 

 and pubic thickenings, though no scales are present. It 

 is probable that the typical form of the reptilian callosity 

 was first determined by the presence of the epidermal 

 scales of the skin, and the latter still responds in the 

 same manner in birds, not only on the legs and toes where 



they are absent The present interest lies in the fact that 

 the characteristic form assumed bv a callous area in the 

 ostrich enables it to be sharplv distinguished from the 



