[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. Frogs of the genus _Plectrohyla_ closely resemble species in the _Hyla bistincta_ group but differ principally in having a projecting prepollex. In the highlands of Costa Rica a group of species, of which _Hyla moesta_ is best known, resembles species in the _Hyla bistincta_ group. At present insufficient information is available on the Costa Rican species to determine their affinities. Analysis of Characters The characters used in the systematic study of the frogs in this group are those usually employed in anuran systematics. Of the various measurements and proportions, the snout-vent length and the relative size of the tympanum to the eye apparently are of more taxonomic importance than the others (Table 1). In all of the species the tympanum is at least partially covered by a heavy, dermal supratympanic fold, and in some specimens of _H. pachyderma_ the tympanum is completely obscured. In two species (_H. bistincta_ and _H. charadricola_) the snout is square, whereas in the other species it is round. The fingers are long and slender in _H. crassa_, _pachyderma_, and _robertsorum_ and somewhat shorter with more webbing in _H. bistincta_ and _charadricola_. Breeding males of _Hyla pachyderma_ have moderately large nuptial spines; the other species have small spines, except _H. charadricola_ in which spines apparently are absent. A well-defined thoracic fold is present in _H. pachyderma_, and a weak fold is present in _H. robertsorum_; the other species lack folds. In all species there is an anal sheath; this sheath is longest in _H. bistincta_, in which the anal opening is directed ventrally at the level of the lower edge of the thighs. TABLE 1.--COMPARISON OF CERTAIN MEASUREMENTS AND PROPORTIONS IN THE SPECIES OF THE HYLA BISTINCTA GROUP. (DATA FROM ADULT MALES; MEANS ARE GIVEN IN PARENTHESES BELOW THE RANGES.) A: Snout-vent length B: Tibia length / Snout-vent length C: Head width / Snout-vent length D: Tympanum / Eye