A new species of Osteocephalus (Anura : Hylidae) from Guyana

A new member of the genus Osteocephalus is described from the Pakaraima mou ntains of western Guyana. This species is the smallest known member of the genus and is probably closely related to O. subtilis. Both share a small si ze (less than 40 mm snout-vent length), large and bulgy eyes directed somew hat rostrally, green bones, smooth and brownish dorsal skin, relatively sho rt and truncate snout, small tympanum, subgular and laterally expanded voca l sac, poorly developed subarticular and supernumerary tubercles, a supra-a nal glandular ridge, and cream-white venter and subocular region. The new s pecies can be distinguished from O. subtilis by the Buff iris (vs black), s maller overall size (32.7 vs 35.8-38.8 mm snout-vent length), relatively la rger toe disks, and less developed foot webbing. The cranium of the new spe cies is well ossified, relatively reduced in width between the orbits, with out an exposed frontoparietal fontanelle and with the anterior arm of the s quamosal extending to about half the distance to the maxillary. The vocal s ac is subgularly poorly developed and possess lateral extensions to the are a behind the jaw angles. Well developed supraocular and suprasquamosal cart ilages give support to the enlarged eyes of this species.