PLATE 21. 



PREORBITAL FOSSiE IN PROTOHIPPUS. 

 COMPARISON OF FACIAL PART OF SKULL IN THREE SPECIES FROM THE UPPER MIOCENE OF NEBRASKA AND SOUTH DAKOTA . 



All figures one-half natural size. 



Little White River, So. Dak. Lower Pliocene 1. Protolrippm simus Gidley, type, Am. Mus. 9820. Lachrymal fossa 



shallow; no malar fossa. 



Niobrara River, Nebraska 2. P. niobrarcnsis Gidley, type, Am. Mus. 10S2S. Lachrymal fossa 



restricted, moderately deep, anterior in position; no malar fossa. 



Little White River, So. Dak. " " 3. P. pcrditus Leidy, neotype (Gidley), Am. Mus. 10838. Juvenile 



skull; lachrymal fossa restricted, rather shallow; no malar 

 fossa. 



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