Although _Osteocephalus_ and _Argenteohyla_ have similar vocal sac structure, they are obviously distinct. The monotypic _Argenteohyla_ is a rather specialized, semifossorial frog (Trueb, 1970b), characterized by smooth skin, moderate-sized digital discs, and a large inner metatarsal tubercle. The general architecture of the skull is not unlike that of _Osteocephalus_; the skulls of both are well roofed, broader than long, and characterized by posterolaterally oriented parasphenoid alae. _Argenteohyla_ bears small, slightly curved prevomerine dentigerous processes in contrast to the large, angular processes of _Osteocephalus_. The skull of _Argenteohyla_ shows specializations, apparently adaptations to its semifossorial mode of existence, which further distinguish the genus from _Osteocephalus_. In comparison with _Osteocephalus_, the cranium of _Argenteohyla_ is slightly depressed anteriorly, the roofing bones extensively casqued, and the palatines robust. Osteologically, _Osteocephalus_ more closely resembles _Phrynohyas_ than either of the other two genera, but _Osteocephalus_ and _Phrynohyas_ are clearly distinct on the basis of their respective vocal sac structure. Like _Osteocephalus_, skulls of the members of the genus _Phrynohyas_ are broader than long, have extensive dermal roofing bones, and have posterolaterally oriented parasphenoid alae. In contrast to _Osteocephalus_, the dentigerous processes of the prevomers are curved, rather than angular in _Phrynohyas_. Furthermore, the latter genus is singularly distinguished from _Osteocephalus_, _Argenteohyla_, and _Trachycephalus_ by having extensively developed parotoid glands that produce a viscous, milky volatile secretion. _Trachycephalus_ is the most readily identifiable of the four genera under discussion. Members of this genus are large frogs with heavily casqued and co-ossified skulls (Trueb, 1970a). The dermal roofing bones bear ornate and characteristic patterns of sculpturing. The medial ramus of the pterygoid does not articulate with the otic capsule, and the parasphenoid alae are laterally, rather than posterolaterally, oriented. A dermal sphenethmoid is present, and the parasphenoid bears odontoids. The basic structure of the skull has many characters in common with both _Osteocephalus_ and _Phrynohyas_. The obvious modifications of dermal roofing bones and of palatal and suspensory elements seem to be specializations adapting members of the genus _Trachycephalus_ to their peculiar phragmotic habits. The vocal sac structure of _Trachycephalus_ is like that of _Phrynohyas_ and therefore further distinguishes it from _Osteocephalus_.