ve a notably different coloration in life. The dorsum is
uniform pale green; the anterior and posterior surfaces of the thighs,
fingers, first three toes, and webbing are deep yellow. The anal stripe
is creamy white, and the flanks are pale gray with black flecks. The
upper lip, supratympanic fold, and canthal stripe are a bronze color.
The belly is pale yellow with a silvery cast on the throat. Juveniles
having snout-vent lengths from 24.5 to 36.6 mm. are so colored in life,
and uniform dark bluish gray dorsally in preservative.

_Comparisons._--The absence of a quadratojugal and the presence of a
greatly enlarged, non-projecting prepollex place _Hyla siopela_ in the
_Hyla bistincta_ group (see Duellman, 1964, and Adler, 1965). The
presence of a rostral keel separates _Hyla siopela_ from other members
of the _Hyla bistincta_ group, which is composed of two species having
long anal sheaths (_bistincta_ and _pentheter_), two small species
having axillary membranes and lacking nuptial excrescences in breeding
males (_charadricola_ and _chryses_), and three species (_crassa_,
_pachyderma_, and _robertsorum_) having short heads, round snouts, short
anal sheaths, and nuptial excrescences in breeding males. _Hyla siopela_
differs from the last three species in the shape of the snout and from
each in certain structural features; _H. crassa_ has fully webbed feet;
_H. pachyderma_ has large nuptial spines, and _H. robertsorum_ has more
webbing and a shorter tarsal fold. Furthermore, the venter in _H.
robertsorum_ is brown with creamy white flecks.

In structure and coloration _H. arborescandens_ resembles _siopela_, but
the former is smaller, and males of _arborescandens_ have vocal slits.

_Remarks._--This description brings to eight the number of species now
recognized in the _Hyla bistincta_ group. _Hyla siopela_ is most closely
related to _Hyla robertsorum_ from the high mountains of the Sierra
Madre Oriental in northern Puebla and eastern Hidalgo. Possibly the four
species now recognized in the _crassa_ subgroup (_crassa_, _pachyderma_,
_robertsorum_, and _siopela_) are only subspecies of a single species,
but differences in the amount of webbing in _crassa_ and the nature of
the nuptial excrescenses in _pachyderma_ indicate that they are distinct
species.

The type locality of _Hyla siopela_ is a small stream cascading down the
western slope of Cofre de Perote; the lower reaches of the stream can be
reached by a dirt ro