New information concerning the ancestry of the later Tertiary horses, Calippus, Protohippus (in the original sense), Hipparion, Neohipparion, and Nannippus, has been obtained from a restudy of the various mammalian farmas of the Coastal Plain in Texas, based in the main on the extensive collections of the Bureau of Economic Geology, The University of Texas. The age, stratigraphic correlation, and composition of these faunas have been reevaluated, and the results provide new evidence that has important bearing on Miocene and Pliocene intercontinental correlations of mammalian faunas. It is the opinion of the writer that these five genera of horses, among which the three genera Hipparion, Neohipparion, and Nannippus are commonly called the Hipparion group, originated from the lower Jv[:iocene genus Parahippus and hence made their first appearance in the middle Miocene.