[Illustration: PLATE V. Fig. 12.—Restoration of _Eohippus_, the American _Hyracotherium_, size about 12 inches. See fig. 10. Fig. 13.—_Orohippus_, a late Eocene four-toed horse, size about 16 inches. See fig. 11. Fig. 14.—_Mesohippus_, an Oligocene horse about 24 inches. In some of the three-toed Oligocene horses the cannon-bones were as long as in a 32-inch Shetland pony. See fig 18. Fig. 15.—_Hypohippus_, a three-toed Miocene “forest” horse, with the II. and IV. toes long and functional. In 40-inch specimens of _Hypohippus_ the cannon-bones were 2·5 inches longer than in some 40-inch Shetland ponies. See figs. 9 and 20. Fig. 16.—_Merychippus_, a 9-hands three-toed Miocene horse. _Protohippus_ (a possible ancestor of modern fine-limbed breeds) and the extinct _Hipparions_ seem to have been derived from _Merychippus_. Fig. 17.—A 33-inch Shetland pony. In modern Shelties the legs are relatively shorter than in the three-toed horses of the later Miocene deposits. Shetland ponies have probably partly sprung from ancestors allied to _Merychippus_ and partly from ancestors with limbs of the _Hypohippus_ type. Figs. 12 to 16 after Osborn and Lull.] [Illustration: PLATE V. Fig. 16.—_Merychippus_, 36 inches. Fig. 12.—_Eohippus_, 12 inches. Fig. 15.—_Hypohippus_, 40 inches. Fig. 14.—_Mesohippus_, 24 inches. Fig. 13.—_Orohippus_, 16 inches. Fig. 17.—Shetland, 33 inches. All the figures one-thirtieth nat. size.] [Illustration: PLATE VI. Fig. 18.—Skeleton of fore-foot of _Mesohippus_. The II., III., and IV. digits are complete, the V. is represented by the upper end of the metacarpal. Oligocene of America. After Marsh. Fig. 19.—Forefoot of _Merychippus_ (or _Protohippus_). Digits II. and IV. shorter than in fig. 18, and the vestige of digit V. very small or absent. American Miocene. After specimen in American Museum of Natural History. Fig. 20.—Forefoot of _Hypohippus_, the Miocene “forest” horse. Digits II. and IV. long as in _Mesohippus_, digits I. and IV. represented by small “splints” not seen in figure. After specimen in American Museum of Natural History. Fig. 21.—Upper molar, _E. stenonis_, natural size. The internal pillar (p) is only one-third the length of the grinding surface of the crown. Pliocene. After Boule. Fig. 22.—Upper molar, _E. fossilis_, natural size. The internal pillar (p) is more than half the length of the grinding surface of crown. Pleistocene. Kent’s Cavern, Devonshire. After Owen. Fig. 23.—Premolar and molars (natural size) of a small mediæval? horse from Aberdour, Aberdeenshire. The internal pillars are short. Small horses with short-pillared teeth have lived in Europe since the end of the Pliocene. In the 36·5-inch Shetland pony Eric the molars very closely agree with those figured. From specimens received from Prof. Arthur Thomson, Aberdeen.