PEEISSODACT YL A. 337 



known members of this family appear at tlie commencement 

 of the Eocene period in North America, and in the Middle 

 and Upper Eocene in Europe. These old types are, how- 

 ever, widely removed in some respects from the modern 

 Horses, and the genus Eqims itself does not appear till the 

 late Miocene or Low^er Pliocene be reached. It is in the 

 North American continent that we find the most complete 

 and instructive series of fossil Equidce ; though no Horses 

 existed in the New World at the time of its discovery by the 

 Spaniards. It is in the fossil forms of this region, also, that 

 we can trace most clearly the line of connected forms through 

 which we may suppose Equus itself to have descended — our 

 knowledge on this subject having been largely increased by 

 the discoveries of Professor Marsh. Many of the fossil 

 Equidce, are still only very partially known, and it will be 

 sufficient here to glance at the more important and better 

 understood types alone. 



The most ancient member of the Equidce at present 

 known is the Eohippus, discovered in the Lower Eocene 

 of New Mexico by Marsh. There is a complete series of 

 teeth, the dental formula being — 



3—3 1—1 4—4 3—3 ^ ^ 



i ; c ; piii ; m = 44. 



The last premolar resembles the premolar in front of it, 

 instead of the molar behind it. The animals comprised 

 in this genus were of smaU size, — the fore-feet possessing 

 four toes, loith a rudimentary thumb in addition ; while the 

 hind -feet have only three toes, all the digits terminating 

 in hoofs. 



In beds slightly higher than those with Eohippus are 

 found the remains of Orohippus. This genus comprises 

 small Equine animals, nearly allied to the preceding, and 

 about as large as Foxes. The fore-feet (fig. 637, a) are 

 four-toed, the third digit being the largest; and the hind- 

 feet are three-toed. There is now no trace of the rudimen- 

 tary poUex of Eohippus ; and the last premolar resembles 

 the molars. The dental formula is the same as in Eohippus, 

 and the canines are large and separated from the premolars 



VOL. II. Y 



