VERTEBRATE LIFE IN AMERICA. 679 



sheep, and one stage nearer the horse. Three are only three toes and 

 a rudimentary splint hone in the fore feet, and three toes behind. 

 Two of the premolar teeth are quite like the molars. The ulna is no 

 lono-er distinct, or the fibula entire, and other characters show clearly 

 that the transition is advancing. In the upper Miocene, Mesohippus 

 is not found, but in its place a fourth form, Miohippus, continues the 

 line. This genus is near the Anchitherium of Europe, but presents 

 several important differences. The three toes in each foot are more 

 nearly of a size, and a rudiment of the fifth metacarpal bone is retained. 

 All the known species of this genus are larger than those of Mesohip- 

 pus, and none pass above the Miocene. 



The genus Protohippus, of the lower Pliocene, is yet more equine, 

 and some of its species equaled the ass in size. There are still three 

 toes on each foot, but only the middle one, corresponding to the single 

 toe of the horse, comes to the ground. This genus resembles most 

 nearly the Hipparion of Europe. In the Pliocene, we have the last 

 stage of the series before reaching the horse, in the genus JPliohippus, 

 which has lost the small hooflets, and in other respects is very equine. 

 Only in the upper Pliocene does the true Eqims appear, and complete 

 the genealogy of the horse, which in the Post-Tertiaiy roamed over 

 the whole of North and South America, and soon after became extinct. 

 This occurred long before the discovery of the continent by Europeans, 

 and no satisfactory reason for the extinction has yet been given. Be- 

 sides the characters I have mentioned, there are many others, in the 

 skeleton, skull, teeth, and brain, of the forty or more intermediate 

 species, which show that the transition from the Eocene Eohippus to 

 the modern Equus has taken place in the order indicated ; and I believe 

 the specimens now at New Haven will demonstrate the fact to any 

 anatomist. They certainly carried prompt conviction to the first of 

 anatomists, who was the honored guest of the Association a year ago, 

 whose genius had already indicated the later genealogy of the horse in 

 Europe, and whose own researches so well qualified him to appreciate 

 the evidence here laid before him. Did time permit, I might give you 

 at least a probable explanation of this marvelous transition, but jus- 

 tice to the comrades of the horse in his long struggle for existence 

 demands that some notice of their efforts should be placed on record. 



Besides the horse and his congeners, the only existing Perissodac- 

 tyles are the rhinoceros and tapir. The last is the oldest type, but the 

 rhinoceros had near allies throughout the Tertiary; and, in view of 

 the continuity of the equine line, it is well worth while to attempt to 

 trace his pedigree. At the bottom of the Eocene, in our Western 

 lake-basins, the tapiroirl genus Selaletes is found, represented by nu- 

 merous small mammals hardly larger than the diminutive horses of 

 that day. In the following epoch of the Eocene, the closely-allied 

 Hyrachyus was' one of the most abundant animals. This genus was 

 nearly related to the Lophiodon of Europe, and in its teeth and skele- 



