The important Oligocene to Miocene tortoise genus Stylemys is redefined. Certain skeletal characters (especially of the type species, S. nebrascensis) believed to be diagnostic of the genus are shown to be interpreted wrongly or based on juvenile specimens. Several new characters are added, and comparisons are made with Gopherus and Geochelone. Validity of European species placed in the genus are discussed. Testudo amphithorax and T. uintensis of the North American Tertiary probably should be assigned to Stylemys.