_Discussion._--The Nitosauridae are small primitive edaphosaurs with a moderately elongate face, sharp subisodont teeth, little development of canines and few specializations. The jaw is of a primitive type and articulates on a level with the tooth-row. The palatal dentition is primitive (Romer, 1956:280). The nitosaurids are thought to be related to the later Caseidae, and the most obvious structural similarities are found in the postcranial skeleton (Vaughn, 1958:989). Cranial resemblances between the families are fewer, but nevertheless indicate that a nitosaurid-caseid relationship exists. Vaughn (1958) described a small pelycosaur, _Colobomycter pholeter_ (Eothyrididae, Ophiacodontia) that structurally resembles the Caseidae. This individual also was obtained from the Fort Sill locality. In Vaughn's opinion the features of _Colobomycter_ indicate a close relationship between eothyridids and caseids and the possibility that the caseids may well have been of eothyridid rather than nitosaurid derivation. In view of this historical uncertainty of the relationships between the Nitosauridae, the Eothyrididae and the Caseidae, it is well to consider how the maxillary fragments described above differ from and resemble representatives of each of these three families as reported in the literature. _Delorhynchus_ resembles _Colobomycter_ in size. The mean extra-maxillary length of the undamaged teeth of the three fragments is 2.5 mm., equal to that reported by Vaughn (1958:985) for teeth about midway in the postcanine series of _Colobomycter_. None of the teeth of _Delorhynchus_ extends beyond the maxillary rim as far as does the canine of _Colobomycter_ (3.5 mm.). The teeth in both genera are conical and sharply pointed. The naris in each is enlarged, and the lacrimal is excluded from the narial margin in each (by inference in _Delorhynchus_.) The differences between the maxillae of _Colobomycter_ and _Delorhynchus_ are most striking in the lack of canines in the latter and the correlated absence of modifications of the maxillary for support of canines. Additionally, _Delorhynchus_ bears an infraorbital canal in contrast to the groove in similar position in _Colobomycter_. The recurvature of the four teeth present in the fragments of _Delorhynchus_ differs from that in the teeth of _Colobomycter_ in which only the canine and precanine are recurved. Vaughn implies that anterior and posterior cutting edges extend the length of the teeth in _Colobomycter_; these are restricted to the distal third of the teeth in _Delorhynchus_. The external surfaces of the maxillae of _Delorhynchus_ are pitted and ridged; Vaughn was unable to discern sculpturing of the corresponding surfaces in _Colobomycter_.