For their willing assistance in the field I am grateful to Ann S. Duellman, Dale L. Hoyt, and John Wellman. Permits for collecting in México were generously issued by the late Ing. Luis Macías Arellano, Departamento de la Fauna Silvestre, Dirección General de Caza. The drawings in figures 1 and 3 were executed by Gail Selfridge. This research has been supported by the National Science Foundation (NSF G-9827).
The five species comprising the _Hyla bistincta_ group are moderate-sized hylids having rather blunt heads and robust bodies. The fingers are long and have little webbing (Fig. 1). The skin of the dorsum is thick and glandular, but not tuberculate. An anal sheath is present. The skull is rather broad, flat, and solidly roofed. The ethmoid is broad, curved downward laterally, and solidly sutured to the frontoparietal. The nasals are broad, sutured for their entire width with the ethmoid, and broadly in contact medially. The premaxillaries are in contact medially; each has a long, flat nasal process. The quadratojugal is absent, and the maxillary tapers to a point posteriorly. There is no squamosal-maxillary connection. The maxillary and premaxillary teeth are rather long, bifid, and moderately spatulate. Some teeth on the premaxillary and anterior part of the maxillary are hooked. The vomerine teeth are spatulate and bifid. A broad, flat, ossified prepollex is present but does not project as a spine. The known tadpoles have ventral mouths, 2/3 tooth-rows, two or more rows of labial papillae, and long tails with low fins.
[Illustration: FIG. 1. Palmar view of right hand of: _a._--_Hyla robertsorum_ (KU 57661), _b._--_Hyla charadricola_ (KU 58414). × 3.]
As thus defined the _Hyla bistincta_ group can be distinguished from all other groups of Middle American frogs by the combination of absence of the quadratojugal, non-projecting prepollex, long fingers with little webbing, and stream-inhabiting tadpoles having 2/3 tooth rows and two or more rows of labial papillae.
Possibly _Hyla arborescandens_ and _Hyla hazelae_ belong in this group. Because these species are somewhat different from the included species and because their tadpoles are as yet unknown, I have refrained from including these two species in the _Hyla bistincta_ group. Taylor (1948:261) assigned _Hyla proboscidea_ (= _H. dalquesti_) and (1949:272) _Hyla cyclomaculata_ to this group, but because these two species have a quadratojugal and notably different tadpoles, they are excluded from the group.