_Syrrhophus_ is closely allied to another Mexican leptodactylid genus, _Tomodactylus_, which was revised by Dixon (1957), who along with numerous other authors noted the close relationship between the two genera. There is an almost complete lack of sympatry between the two genera; in very few places in Mexico do they coexist (Fig. 21). _Tomodactylus_ has its greatest diversity in the Cordillera Volcanica and Sierra Madre del Sur, whereas _Syrrhophus_ reaches its greatest diversity in the Sierra Madre Oriental and eastern foothills. The species of both genera are about the same size and presumably have similar requirements insofar as food, breeding sites, and habitat selection. Four cases of intergeneric sympatry are known for the two genera: 1) the Chilpancingo region of Guerrero, 2) the lowlands of Colima and the mountains just inland in Jalisco, 3) the lowlands of central Nayarit, and 4) the Sierra Madre Occidental on the Durango-Sinaloan border. The apparent sympatry in the Chilpancingo region involves four species: _S. pipilans_, _T. albolabris_, _T. dilatus_, and _T. nitidus_. Of the four, _T. dilatus_ appears to be completely allopatric in that it occurs at higher altitudes (above 2000 meters), whereas the other three occur below 1800 meters in the region (Davis and Dixon, 1965). In the Colima-Jalisco region, _Tomodactylus_ tends to occur higher (Dixon and Webb, 1966) than some of the _Syrrhophus_, but one subspecies of _Tomodactylus nitidus_ is a lowland frog, occurring sympatrically with the lowland _Syrrhophus modestus_. A similar situation is observed in Nayarit; the lowland _Tomodactylus_ occurs sympatrically with the small _Syrrhophus pallidus_. In both cases the _Syrrhophus_ is smaller than the _Tomodactylus_. [Illustration: FIG. 22: Altitudinal distributions of _Syrrhophus_ and _Tomodactylus_. Widths of the columns are proportional to the numbers of species at a given altitude; narrowest width equals one species.] Frogs of the genus _Syrrhophus_ tend to occur at lower elevations than do their close relatives of the genus _Tomodactylus_ (Fig. 22). This generalization is complicated by the occurrence in the Sierra Madre Oriental in relatively high altitude _Syrrhophus_ (up to 2000 m.) and the occurrence in Michoacan of low altitude _Tomodactylus_ (to sea level). There are no _Tomodactylus_ in the Sierra Madre Oriental, whereas the genus _Syrrhophus_ is represented in the lowlands of western Mexico (_modestus_ group). _Syrrhophus_ and _Tomodactylus_ exhibit essentially parapatric distributions. The two genera as now composed can be characterized as low to moderate elevation frogs (_Syrrhophus_) and moderate to intermediate elevation frogs (_Tomodactylus_). LITERATURE CITED BAIRD, S. F. 1859. Reptiles of the Boundary. United States and Mexican Boundary Survey, pp. 1-35, pls. 1-41. BARBOUR, T.