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THE AMERICAN NATURALIST [Vol. LI V 



tarsals and feet (Fig. 2). The sternal and pubic callosi- 

 ties may therefore be looked upon as a direct response 

 of the skin to the pressure and friction of the body 

 against the hard ground. Also in its frequent habit of 



taking a 1 ' dust-bath'' the ostrich rolls from side to side, 

 the two projections being in the axis of motion, and this 

 serves further to extend the area subject to pressure and 

 friction. 



In man and mammals generally a callosity usually con- 



