0. C. Marsh — Recent Polydactyly Horses. 353 



The dividing lines between these three families are not 

 sharply defined, and cannot be, if evolution is true ; in fact, 

 the divisions in all classifications of extinct animals are, of 

 necessity, like the book shelves in a library, merely convenient 

 stages for the arrangement of present limited knowledge. 



Among the extinct equines known in this country, the first 

 genus of the series is Eohipjpus, described by the writer from 

 the Coryphodon beds at the base of the Eocene (this Journal, 

 vol. xii, p. 401, 1876). This is the oldest and most diminutive 

 form, being about the size of a small fox. There is a diastema 

 between the canine and first premolar, but none between the 

 premolars, which are all unlike the molars. Three species are 

 known. The feet of one species are shown in figures 14 and 

 15, and those of the others so far as known are similar. 



In the next higher horizon of the Eocene, remains of another 

 small equine mammal are found, which may be placed in a new 

 genus, Helohippus. The type specimen was described by the 

 writer in 1871, as Lophiodon pumilus (this Journal, vol. ii, p. 

 38), and others have since been found in the same horizon. 

 This genus has a diastema between the first and second pre- 

 molars, and the last premolar is like the molars. The known 

 representatives of Helohippus are about as large as a fox. 



The next genus in the series is Orohippus, described by the 

 writer from the Dinoceras beds of the Middle Eocene (this 

 Journal, vol. iv, p. 207, 1872). This form is the type of the 

 family Orohippidce, and almost every part of the structure of 

 several species is now known. The best preserved specimens 

 show that there is no diastema between the upper premolars, 

 and the third and fourth are similar to the molars. The skull 

 is elongate, and has no antorbital fossa. The fore and hind 

 feet of one species are shown in figures 16 and 17. Several 

 other species are known, all about the size of a fox. 



Another allied genus, JEpihvppus, occurs in the Diplacodon 

 beds of the Upper Eocene. This genus, proposed by the 

 writer in 1878 (Proc. Am. Assoc. Adv. Sci., page 236), also 

 shows no diastema between the first and second premolars 

 above or below, and the third and fourth are essentially like 

 the molars. Two species are known, both somewhat larger 

 than those of Orohippus, but having the same general struc- 

 ture, although the dentition and feet differ in several respects, 

 being intermediate between that genus and the following. 



At the base of the Miocene, in the Brontotherium beds, 

 another equine mammal is comparatively abundant, and has 

 been referred by the writer to a distinct genus, Mesohippus 

 (this Journal, vol. ix, p. 248, 1875). The two species known 

 are about as large as a sheep, and the feet of one are shown in 



