New Equiiie Ifammals from the Tertiary Formation. 1513 



genera are very similar, but not so manifest as the oldest type (Oro- 

 hippus), as the latter is by its Miocene relative. A still older ances- 

 tor, probably the ci'etaceous, doubtless had five toes in ea(;li foot, the 

 typical number in mammals. This reduction in the number of toes 

 may, perhaps, have been due to elevation of the region inhabited, 

 which gradually led the animals to live on higher ground, instead of 

 the soft lowlands, where a po'ydactyl foot would be an. ad vantage. 



The gradual elongation of the head and neck, which took place in 

 the successive genera of this group during the Tertiary period, was a 

 less fundamental change than that which resulted in the reduction of 

 the limbs. The process may be said to have already began in 

 Orohipjnts, if we compare that form with other most nearly allied 

 mammals. The diastema, or " place for the bit," was- well developed in 

 both jaws even then, but increased materially in succeeding genera. 

 The number of the teeth remained the same until the Pliocene, when 

 the front lower jiremolar was lost and subsequently the corresponding 

 upper tooth ceased to be functionally developed. The next upper 

 premolar, which in Orohippus was the smallest of the six posterior teeth, 

 rapidly increased in size and soon became, as in the horse, the largest 

 of the series. The grinding teeth at first had very short crowns, with, 

 out cement, and were inserted by distinct roots. In Pliocene species, 

 the molars became longer and were more or less coated with cement. 

 The canine teeth were very large in Orohippus, and in this genus, as 

 well as those from the Middle Tertiary, appear to have been well devel- 

 oped in both sexes. In later forms these teeth declined in size, espe- 

 cially as the changes in the limbs afforded other facilities for defense, 

 or escape from danger. The incisors in the early forms were small, 

 and without the characteristic pit of the modern horse. In the genera 

 from the American Eocene and Miocene the orbit was not inclosed 

 behind by an entire bridge of bone, and this first makes its appearance 

 in this country in Pliocene forms. The depression in front of the orbit, 

 so characteristic of Anchitherlum and some of the Pliocene genera, is, 

 strange to say, not seen in Orohippus, or the later Miohippus, and is 

 wanting likewise in exisihg horses. It is an interesting fact that the 

 peculiarly equine features acquired by Orohippus are retained persist- 

 ingly throughout the entire series of succeeding forms. Such, e. g., is 

 the form of symphysial part of the lower jaw, and also the charac- 

 teristic astragalus, with its narrow, oblique, superior ridges, and its 

 small articular facet for the cuboid. 



Such is, in brief, a general outline of the more marked changes that 

 seem to have produced in America the highly specialized modern Eqicus 

 from his diminutive, four-toed predecessor, the Eocene Orohijyjms. The 



