_†Lambdotherium_, low. Eoc. _†Eotitanops_, low. Eoc. _†Palæosyops_, mid. Eoc. _†Telmatherium_, mid. Eoc. _†Dolichorhinus_, up. Eoc. _†Diplacodon_, up. Eoc. _†Titanotherium_, low. Oligo. III. TAPIRIDÆ. Tapirs. _†Systemodon_, low. Eoc. _†Isectolophus_, mid. and up. Eoc. _†Protapirus_, Oligo. _†Tapiravus_, mid. Mioc. _Tapirus_, Pleist., N. Amer., Pleist. and Recent, S. Amer. IV. †LOPHIODONTIDÆ. †Lophiodonts. _†Heptodon_, low. Eoc. _†Helaletes_, mid. Eoc. _†Colodon_, low. Oligo. V. RHINOCEROTIDÆ. True Rhinoceroses. _†Trigonias_, low. Oligo. _†Cænopus_, Oligo. and low. Mioc. _†Diceratherium_, up. Oligo. and low. Mioc. _†Aphelops_, mid. Mioc. to low. Plioc. _†Teleoceras_, mid. Mioc. to low. Plioc. VI. †HYRACODONTIDÆ. †Hyracodonts and †Amynodonts, cursorial and aquatic Rhinoceroses. _†Hyrachyus_, low. and mid. Eoc. _†Triplopus_, mid. and up. Eoc. _†Colonoceras_, mid. Eoc. _†Hyracodon_, low. Oligo. _†Amynodon_, up. Eoc. _†Metamynodon_, low. Oligo. Suborder †ANCYLOPODA. †Clawed Perissodactyls VII. †CHALICOTHERIIDÆ. Chalicotheres. _†Moropus_, up. Oligo. and low. Mioc. _?†Schizotherium_, low. Oligo. _†Eomoropus_, mid. Eoc. The earliest perissodactyls of which we have any knowledge are found in the older part of the lower Eocene (Wasatch stage) of Europe and North America, into which they must have migrated from some other region yet unknown, for no probable ancestors of the group are found in the Paleocene of either continent. I. SUBORDER CHELODACTYLA. NORMAL PERISSODACTYLA. 1. _Equidæ. Horses_ In order to make intelligible the evolutionary changes which have led up to the modern horses, it will be necessary to say something concerning the dental and skeletal features which characterize these animals. Using the term _horses_ in a broad sense to include all the existing members of the family Equidæ, true horses, asses, zebras and quaggas, we find a greater uniformity in the skeleton and teeth than would be expected from the external appearance. The differences in appearance are, however, largely due to colouring, growth of mane and tail and the size of the ears, which leave no record in the skeleton. [Illustration: FIG. 147.—Asiatic Wild Horse (_Equus przewalskii_).—By permission of the N.Y. Zoölog. Soc.] The teeth (Figs. 45, p. 95; 154, p. 306) are extremely high-crowned, or hypsodont, and do not form roots till an advanced age; the incisors have a deep, enamel-lined pit, the “mark” in the centre of the grinding surface; the first premolar in each jaw is very small and of no functional importance; the other premolars have the same pattern as the molars, which is excessively complex in the arrangement of the enamel ridges and the areas of dentine and cement.