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University of California Publications. 



[Geology 



species was very difficult. As shown by Gidley the lower milk 

 molars of Hypohippus, as represented by this form, are dis- 

 tinguished from those of Mesohippus by the heavier and better 

 developed external basal cingula, the protoconid and hypoeonid 

 being fuller and wider transversely, and the teeth more special- 

 ized in general. Especially is the advanced development notic- 

 able in lower milk molar two in Hypohippus "in which the 

 anterior external cusp has attained a completely crescentic form 

 similar to that of the posterior cusp, while in Mesohippus this 

 tooth has but one crescent, or V, the posterior one." 



M lt the only permanent tooth represented in specimen 10665, 

 described by Gidley, is slightly larger than the teeth of H. 

 equiuus and is a little smaller than in H. affinis. It is of almost 



Figs. 28a and 286. Hypohippus, near osborni Gidley. Lower jaw with 

 dentition. No. 12587, X %. Virgin Valley Beds, Virgin Valley, Nevada. 

 Tig. 28a, outer side; fig. 28b, superior view. 



