EQUIDA

the horse family, a division of the odd-toed (peri odactyle) Unguiates or hoofed mammals. There is but one existing genus, Equus, distinguished by the po e ion of a mane; hard pads (callosities) on the inner side of each fore-limb (and sometimes of the hind-limb); a single functional digit (the third or middle one) terminating in a large curved hoof; a simple stomach; 44 teeth, including 12 incisors with pitted crowns, eanines (tushes) reduced in the female, and 28 grinding teeth with broad crowns, except the first (wolf tooth), which is rudimentary. The forms included are horses, a es, and zebras; the first being distinguished from the others by the presence of callosities (‘chestnuts’) on the hind-limbs as well as the fore. The domesticated horse (#. cab all i us) has a large flat tail abundantly hairclad, and is not known with certainty in the wild state, though po ibly the tarpan of South Ru ia (Tartary) may represent the original stock. Another candidate for this honour is the small wild species (2. przewalskii) native to the deserts of Central Asia. The domestic a (#. asinus) is related to a number of wild species, such as the onager (2. onager) of South Asia, the kiang (2. hemionus) of Tibet, and two African species (2. african us and #. The striped zebras are purely African, and four species are generally recognized—the common or mountain zebra (#. zebra), Burcheli’s 4 208 EQUILIBRIUM a (#. burchelli), Grevy’s zebra (#, grevyi), and the quagga (#. quagga). The geological record enables us to derive horses from a small plantigrade ve-toed form (Phenacodus), by gradual increase in size, complication of teeth, lo of digits, and elongation of limbs, to the unguligrade condition.