up 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). x 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.

Analy