S. S. Lull — The Evolution of the Horse Family. 173 



In the rocks of the Bridger age two genera of horses are 

 found, Helohippus and Orohippus, the former being known 

 only by the teeth. The type specimen of H. pumilus Marsh 

 is in the Yale collection. Orohippus, on the contrary, is well 

 known, and represents the fourth stage in the evolutionary 

 series. It is but little advanced over Protorohippus, which it 

 somewhat exceeds in size. The foot structure is much the 

 same, but the teeth show the most notable improvement over 

 those of its predecessor. The third premolar has assumed the 

 full molar form and the second is beginning to do so. The 

 space, or diastema, between the cropping and grinding teeth, is 

 rapidly increasing, owing to the lengthening of the jaws. 



The genus Orohippus includes a number of species from 

 Wyoming and New Mexico, the Yale collection containing 

 the type specimens of O. agilis, 0. oallardi, O. major, O. 

 pumilus, and O. uintanus, besides numerous other specimens. 

 That of Orohippus agilis is especially fine, consisting of a 

 skull, fore foot, and other parts of the skeleton, while a lower 

 jaw and hind foot are probably also referable to this species. 



The final Eocene age, that of the Uinta, brings into view 

 the fifth, or Epihippus, stage in the horse series, but unfortu- 

 nately one not well known. Curiously enough, the average 

 size of the species seems somewhat less than that of the pre- 

 ceding genus, although the Yale collection contains a hind 

 foot and other portions of an unnamed form of much larger 

 size, rivaling that of Mesohippus of the Oligocene. Epihip- 

 pus has four toes in front and three behind, but the lateral 

 ones are becoming shorter and bear less of the creature's 

 weight than formerly. The premolar teeth are all molariform 

 with the exception of the first, and the crests are almost com- 

 pletely formed. But two species of this genus have been 

 described, both by Professor Marsh, the types being in the 

 Yale collection. They are Epihippus gracilis and E. uinten- 

 sis from Utah. 



OLIGOCENE PERIOD. 



During Oligocene times, much the same conditions pre- 

 vailed as in the Eocene. The drying up of streams and lakes, 

 due to increasing aridity of climate, gave great impetus to the 

 development of broad meadow lands and to the true prairie as 

 well. Thus there were three conditions, — woodland, meadows, 

 and dry prairie, which seem to have given rise to several 

 parallel lines of evolution, some of which terminated, being 

 overcome in the struggle for existence, while others flourished 

 and gave rise to the horses of the Miocene. 



But two genera of Oligocene horses are recognized, Meso- 

 hippus and Miohippus, the former being the more primitive, 



