Vol. 5] Qidley. — Fossil Mammals from Virgin Valley. 



237 



unworn. A second important difference is the relatively greater 

 depth of the lower jaw, which further suggests that this specimen 

 may after all represent a new species. 



The lower milk-clentition of species of Hypohippus has not 

 hitherto been known, hence the deciduous teeth of the present 

 species can be compared only with the permanent series and with 

 those of other genera where they are known. The lower milk 

 molars of the present species present in general the characters 

 distinctive of the genus. As in other species of horses, the crowns 

 are relatively lower and narrower transversely than those of the 

 permanent series. 



The more important characters which distinguish the milk- 

 molars of Hypohippus from those of Mesohippus are as follows: 

 They have a heavier and better developed external basal eingu- 

 lum, the two outer cusps, pr A and hy d , are fuller and wider trans- 

 versely, and the teeth are more specialized in general than in 

 species of Mesohippus. Advanced development in Hypohippus 

 is especially marked in dp., in which the anterior external cusp 

 has attained a completely creseentic form similar to that of the 

 posterior cusp, while in Mesohippus this tooth has but one cres- 

 cent, or V, the posterior one. In this respect dp 2 of Hypohippus 

 is more highly specialized than p., of the permanent series of 

 either Mesohippus or Hypohippus and resembles the p 2 of Para- 

 hippus. The metaconid-metastylid column is broader antero- 

 posteriorly than in the permanent molariform teeth of the genus, 

 and these cusps are slightly but distinctly separated at the sum- 

 mit of the column. 



Compared with the corresponding tooth, dp 2 , associated with 

 the type of Parahippus cognatus, that of Hypohippus presents 

 the following characteristic differences: (1) The outer basal 

 cingulum is much better developed and is continuous, while it is 

 entirely interrupted on the external walls of the protoconid and 

 hypoconid in P. cognatus; (2) the summits of the outer cusps, 

 ky a and pr'\ are situated relatively nearer the outer side of the 

 crown, giving a less abrupt slope to the outer walls of the internal 

 valleys; (3) the cusps of the inner row are less well developed 

 throughout; and (4) the metastylid is much less prominent and 

 not separated from the metaconid except slightly at the extreme 



