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.

Delorhynchus resembles the nitosaurids in size, the shape and sharpness of the teeth, their recurvature and the slight enlargement of their bases, the exclusion of the lacrimal bone from the narial margin (in Mycterosaurus) and the apparent lack of a special canine pair of teeth. Resemblances to the caseids are to be noted in the enlargement of the naris (4.5 mm. in height as opposed to 1.7 mm. in Colobomycter), lack of development of canines, presence of an infraorbital canal (in Cotylorhynchus) and absence of many replacement gaps in the marginal row of teeth.

The absence of caniniform enlargement and the extension of the maxilla dorsad of the naris exclude Delorhynchus from the Eothyrididae (Ophiacodontia) but are no bar to its inclusion in the Nitosauridae (Edaphosauria). The marginal teeth of Delorhynchus are simple and primitive, being much like those of the nitosaurids that are described in the literature.

The large narial opening and its posterior, dorsal and ventral enclosure by the maxilla, the infraorbital canal, and the sculptured external surfaces of the maxillary fragments indicate that Delorhynchus, in these features at least, is close to achieving the caseid grade.

Family SPHENACODONTIDAE

Thrausmosaurus serratidens new genus and new species

(Thrausmosaurus is formed from the neuter Greek noun, thrausma, meaning fragment, and the masculine Greek noun, sauros, meaning reptile. The specific name, serratidens, is formed from the Latin serratus, meaning serrate, and the masculine Latin noun, dens, meaning tooth. The specific name is used as a substantive in apposition with the generic name.)

Type specimen.—Fragmentary left dentary, bearing five teeth, the most posterior of which is broken at the