_Cyclagras_, _Hypsirhynchus_, and _Pseudablabes_ have round pupils and certain differences in scutellation. _Drepanoides_ and _Tachymenis_ have elliptical pupils like those of _Tantalophis_, but _Tachymenis_ has only one apical pit, and _Drepanoides_ has one apical pit or none. In the above characters no especially close relationship between _Tantalophis_ and any one of these genera is apparent. If the characteristics usually employed in distinguishing and relating genera are ignored and other more subjective criteria are used, the relationships of _Tantalophis_ still remain obscure. Of the xenodontine genera _Tantalophis_ approaches _Leimadophis_ in general physiognomy; perhaps it represents an early divergent stock of _Leimadophis_ that has undergone radical changes in the hemipenis and other characters. On the other hand, if the nature of the hemipenis is of no importance in defining supergeneric groups of colubrid snakes, _Tantalophis_ may have its relationships with _Leptodeira_ and _Hypsiglena_. Although herpetologists have been working intensively on American colubrids for many decades, the relationships of the majority of the groups are not well understood. Until the hemipenes and skulls of all of the forms have been studied and compared, and the evolutionary significance has been determined for the characters of the hemipenes, dentition, and apical pits, our knowledge of the relationships of these snakes will be incomplete. _Remarks._--The individual on which this paper is based is the only specimen of the species with definite locality data. It is from a locality six miles southeast of Tamazulapam in northwestern Oaxaca. This town lies at an elevation of about 6500 feet in the upper reaches of the Balsas Basin, an arid interior valley that expands in its upper end to form a broad basin of rolling and dissected terrain ranging from about 4000 to 6800 feet in elevation. The countryside around Tamazulapam is arid and supports plants of the genera _Prosopis_, _Acacia_, _Ipomoea_, and _Cassia_, and also columnar cacti. Oaks and pines grow on the limestone hills rising above the rolling valley. _Tantalophis_ may be endemic to the Balsas Basin, as are many other species of reptiles. However, if the snake has its relatives to the south in lower Central America and South America, such a distribution seems unlikely, even for an apparent relict. _Acknowledgments._--For permission to study and report on this specimen I am indebted to Dr. E. Raymond Hall and Mr. John M. Legler. I am grateful to Dr. Laurence C. Stuart for many helpful suggestions and to Dr. Norman E. Hartweg for placing at my disposal the facilities of the Museum of Zoology at the University of Michigan. LITERATURE CITED AMARAL, A. DO 1930 Estudos sobre ophidios neotropicos XVIII--Lista remissiva dos ophidios da regiao neotropica. Mem. Inst. Butantan, 4:129-275. BOULENGER, G. A.