The affinities of _Ptychohyla_ apparently are not with any of the
other groups that have been generically separated from _Hyla_. Of the
daughter genera in Middle America only _Plectrohyla_ has
stream-adapted tadpoles, but these large frogs are not closely related
to _Ptychohyla_. Stuart (1954:169) suggested that certain montane
species of _Hyla_ in lower Central America and _Hyla salvadorensis_ in
El Salvador may be related to _Ptychohyla_ or even congeneric. I have
had experience with most of these species in the field and believe
that Stuart was correct in his suggestion of relationships. The
species concerned are four red-eyed stream-breeding _Hyla_ in Costa
Rica--_H. alleei_, _H. legleri_, _H. rufioculis_, and _H. uranochroa_,
plus _Hyla salvadorensis_ in the mountains of El Salvador.
Morphologically all of the species are similar; _Hyla uranochroa_, _H.
legleri_, and _H. rufioculis_ have a lateral white stripe that is
expanded to form a spot beneath the eye, as in _Ptychohyla
schmidtorum_. The tadpoles of _Hyla rufioculis_ and _H. uranochroa_
have large funnel-shaped mouths and long slender tails like those of
_Ptychohyla schmidtorum_. Lips of the tadpoles of _H. legleri_ and _H.
salvadorensis_ are folded laterally, in this respect resembling those
of the _Ptychohyla euthysanota_ group. I do not know the tadpoles and
the breeding call of _Hyla alleei_. The breeding calls of _Hyla
rufioculis_ and _H. uranochroa_ consist of high melodious notes; the
calls of _H. legleri_ and _H. salvadorensis_ consist of series of
short notes that have the general characteristics of the call of
_Ptychohyla schmidtorum_. Affinities of the genus _Ptychohyla_ seem to
me to be with the red-eyed species forming the _Hyla uranochroa_ group
in Costa Rica. All of the species in the _Hyla uranochroa_ group have
large frontoparietal fontanelles, rather small ethmoids, and small
nasals that are not in contact with one another or with the ethmoid.
Some species have a complete quadratojugal-maxillary arch; others do
not. Assuming that the parental stock that gave rise both to the _Hyla
uranochroa_ group and to _Ptychohyla_ was widespread in Central
America at a time of cooler, more humid conditions, it is possible
that with subsequent warming temperatures and seasonal rainfall in the
lowlands the parental stock was restricted to the Costa Rican
highlands, where the _Hyla uranochroa_ group developed, and to the
Chiapas-Guatemala highlands, where _Ptychohyla_ evolved.