According to Tyler (pers. com.), _Osteocephalus_ has three distinctive types of vocal sac structure which result from differences in the development of the interhyoideus muscle and the overlying skin. In _leprieurii_ and _verrucigerus_ the supramandibular portions of the interhyoideus form a simple tubular, posterolateral extension; there is no modification of the associated skin. _Osteocephalus buckleyi_ and _pearsoni_ have more extensive development of the supramandibular portions of the interhyoideus; furthermore, the associated skin forms a broad, loose fold extending from the ventromedial surface of the throat dorsolaterally to the base of the supratympanic fold. Like _buckleyi_ and _pearsoni_, the supramandibular portion of the interhyoideus is much expanded in _taurinus_. The vocal sac structure of _taurinus_ differs from that of other members of the genus in that the skin of _taurinus_ forms an everted pouch, which dangles loosely beneath the supratympanic fold. KEY TO THE SPECIES OF _Osteocephalus_ 1. Inner edge of third finger webbed to mid-length of antepenultimate phalange; dorsum brown with dark brown spots or median blotch; skull in adults casqued and co-ossified with prominent supraorbital flanges _O. taurinus_ Inner edge of third finger webbed to base of antepenultimate phalange; dorsum plain or marked with dark blotches or transverse bars; skull in adults smooth or slightly exostosed, lacking supraorbital flanges 2 2. Skin on flanks areolate; dorsum in males bearing a mixture of large and small non-spinous tubercles; lips distinctly barred _O. buckleyi_ Skin on flanks smooth; dorsum in males bearing tubercles of uniform size; lips not barred 3 3. Dorsal pattern consisting of narrow transverse dark bars; dorsum in males bearing numerous small spinous tubercles _O. leprieurii_ Dorsal pattern not consisting of transverse bars; dorsal tubercles large or few in number 4 4. Dorsum uniformly dark brown; venter heavily mottled with black, especially in females; dorsum in males bearing large, keratinized tubercles _O. verrucigerus_ Dorsum tan with irregular dark brown blotches; venter cream with fine brown reticulations; dorsum in males bearing few, small non-spinous tubercles _O. pearsoni_ ACCOUNTS OF SPECIES =Osteocephalus buckleyi= (Boulenger)