Endocranial casts of fossil Equidae and the extant horse are described. The series of brain forms from the Lower Eocene Eohippus to the Recent Equus demonstrates the evolution of the brain in an established ancestry. The outstanding feature is increasing predominance of the cerebrum, with expansion particularly of the neocortex; but in many details the process of phylogenetic evolution during this period of about 55,000,000 years took a course different from that postulated by the current conceptions of mammalian brain evolution which are deduced from the conditions in extant lower and higher mammals. The brain of the earliest representative of this family of the higher mammals was of a type which today is characteristic of the lowest marsupials. The type of brain characterizing ungulates today was not achieved before the late Miocene. In its development, progress was not in every phase linked with either increase in body size or skeleton specialization.