The promylagaulines are represented in the late Hemingfordian to early Barstovian by a single genus, Galbreathia n. gen. Galbreathia is erected for the problematical Hemingfordian species Mylagaulus novellus Matthew. The early Barstovian species Trilaccogaulus bettae Sutton and Korth is included as a referred species. This genus is included in the generally primitive Promylagaulinae rather than the advanced Mylagaulinae because of the primitive condition of the skull (not as wide as long, single sagittal crest, more gracile appearance, occipital not anteriorly sloped) and skeleton (limb bones not as fossorially adapted), even though it has attained many of the derived features of the cheek teeth of Mylagaulinae (loss of P3, greater hypsodonty, loss of first molar with eruption of permanent premolars). Galbreathia is a derived promylagauline that shows parallel development of the dentition with the later, more specialized mylagaulines.