_Terrapene ornata_ is a characteristic inhabitant of the western prairies of the United States, and ranges from western and southern Illinois, Missouri, Oklahoma, and all but the extreme eastern part of Texas, westward to southeastern Wyoming, eastern Colorado, eastern and southern New Mexico, and southern Arizona, and, from southern South Dakota and southern Wisconsin, southward to northern Mexico (Fig. 1). It is the only species of the genus that occurs in both Mexico and the United States. The northeasternmost populations of _T. ornata_, occurring in small areas of prairie in Indiana and Illinois, seem to be isolated from the main range of the species. The ranges of _T. ornata_ and _T. carolina_ overlap in the broad belt of prairie-forest ecotone in the central United States. Interspecific matings under laboratory conditions are not uncommon and several verbal reports of such matings under natural conditions have reached me. Nevertheless, after examining many specimens of both species and all alleged "hybrids" recorded in the literature, I find no convincing evidence that hybridization occurs under natural conditions. _Terrapene ornata_ differs from _T. carolina_ in having a low, flattened carapace lacking a middorsal keel (carapace highly arched and distinctly keeled in _carolina_), and in having four claws on the hind foot (three or four in _carolina_), the claw of the first toe of males being widened, thickened, and turned in (first toe not thus modified in _carolina_). _Terrapene ornata_ is here considered to be the most specialized member of the genus by virtue of its reduced phalangeal formula, lightened, relatively loosely articulated shell, reduced plastron, and lightly built skull, which completely lacks quadratojugal bones (Fig. 2); most of these specializations seem to be associated with adaptation for terrestrial existence in open habitats. [Illustration: FIG. 1. Geographic distribution of _Terrapene ornata_. Solid symbols indicate the known range of _T. o. ornata_ and hollow symbols the known range of _T. o. luteola_. Half-circles show the approximate range of intergradation between the two subspecies. Triangles indicate localities recorded in literature; specimens were examined from all other localities shown. Only peripheral localities are shown on the map.]