Probably no ecological group of hylid frogs (some Hyla plus Plectrohyla and Ptychohyla) in Middle America is so poorly known as those species that live in the cloud forests on steep mountain slopes and breed in cascading mountain streams. During the last half of the nineteenth century most of the species of hylids living in the lowlands of southern México and northern Central America were named and described. Despite the extensive collecting by Salvin and Godman, Nelson and Goldman, and the various expeditions of the Mission Scientifique, no members of the genusPtychohyla were obtained until 1927, when in the mountains of El Salvador Ruben A. Stirton found a small tree frog that subsequently was described and named Hyla euthysanota by Kellogg (1928). Until recently frogs of this genus were known from few specimens and in the literature by nearly as many names.

Although I first collected Ptychohyla in 1956, it was not until 1960 that special efforts were made to obtain specimens of this genus. The summer of 1960 was spent in southern México and Guatemala, where every accessible stream in the cloud forests was searched for tree frogs, especially Ptychohyla and Plectrohyla. Similar, but less extensive, investigations were carried out in 1961 and 1962. The result of this field work is a rather large collection ofPtychohyla representing all of the known species, plus tape recordings of the breeding calls and tadpoles of all of the species.

Previously, I have discussed the nomenclature of one of the species (Duellman, 1960) and have described two new species (Duellman, 1961). In the latter paper I made reference to a future account (this one) that would deal with the systematics and biology of the entire genus. Although I have series of specimens, tadpoles, osteological preparations, and recordings of breeding calls, thereby having a wide array of data at my disposal, much still remains to be learned about these frogs, especially about various aspects of their life histories. Even the validity of the genus is open to question; this problem is discussed at length in the section beyond entitled "Ptychohyla as a Natural Assemblage."