SUMMARY. 1The holotype of H. vulpiceps, nearly complete and uncrushed, is the best known skull and jaws of Hyracotherium. It is refigured to show the structure more clearly than in previous published illustrations. 2The holotype of H. leporinum, also uncrushed but fragmentary, reveals foramina and other details of the orbital wall. Exact homologies are dubious, in part, but the various possibilities are pointed out. In any case there are significant differences from Equus. 3Teeth of hyracotheres are extraordinarily variable. No two specimens are in really close agreement. This is especially well exemplified by P3, which seems not yet to have been fully brought under a unified field control of the cheek-tooth series. 4Specimens from the London Clay do not suffice to show whether their strong variation was segregated into two or more separate specific populations or was manifested in a series of interbreeding populations, perhaps with some progressive temporal change. The two supposed species, H. leporinum and H. vulpiceps, have not been soundly differentiated or defined, and, apart from the types, no specimen can be confidently placed in one species rather than in the other. The older species, H. cuniculus, from Kyson, is probably distinct but also lacks adequate definition. Specimens from the Blackheath Beds are more like those from the London Clay, but are also impossible to allocate soundly as to specific or subspecific relationships. 5There are no constant or surely signifimnt structural differences between British and Amercian early Eocene hyracotheres as now known, and Forstar-Cooper must be followed in considering Eohippus as a synonym of Hyracotherium. “Eohippus” might well be retained as a vernaculer name for Hyracotherium. 8British specimens of Hyracotherium are not, as a group, more primitive than American specimens. Most British specimens are about as advanced as those found in the early Eocene of the United Statea. The perhaps most primitive British specimens are not clearly more primitive than the oldest American forms, and some London Clay specimens seem to compare best with rather advanced, late Wasatchian, American forms.