Prionodactylus differs from Euspondylus in having a double row of widened gular scales and keeled hexagonal scales; it is therefore removed from synonymy with Euspondylus. Five species (P. vertebralis, P. dicrus new species, P. manicatus, P. argulus, and P. eigenmanni) are recognized. P. dicrus is related to P. vertebralis; in both, the loreal is separated from the supralabials, the subdigital lamellae are not tuberculate, and the frontonasal is not divided; P. dicrus differs in having two dorsolateral light stripes anteriorly that fuse to one posteriorly, rather than a single median light line throughout. P. vertebralis is known from Pacific Ecuador and Colombia, the Magdalena and Cauca valleys of Colombia, and from adjacent Panamá and Venezuela; Euspondylus ampuedae appears to be the same species (new synonymy). P. dicrus occurs on the Amazonian slopes of central Ecuador. P. manicatus has two subspecies. P. m. manicatus, with single tubercles on the subdigital lamellae, no median collar scale, and a divided eyelid disc, occurs in Amazonian Ecuador and northern Peru. P. m. bolivianus, with non-tuberculate subdigital lamellae, a median collar scale, and an undivided eyelid disc, occurs in Amazonian Bolivia and southern Peru. In both races, the loreal touches the supralabials and the frontonasal is not divided. P. argulus has a wide range in the Guianas and Amazonian South America, at relatively low elevations, from northern Brazil along the slopes of the Andes to Bolivia. It has a divided frontonasal, and the loreal touches the supralabials. P. eigenmanni occurs only in Amazonian Bolivia. It has a single frontonasal, the loreal touches the supralabials, and the subdigital lamellae may have both single and double tubercles. Prionodactylus spinalis and P. rahmi are placed in the genus Euspondylus because they do not have a double widened row of gular scales. Prionodactylus ocellifer is identified as Aspidolaemus affinis (new synonymy). Riolama, new genus, is proposed for Prionodactylus leucostictus. Riolama differs from Prionodactylus in not having a double row of widened gular scales, in having a superficial tympanum rather than a recessed one, in having much depressed digits, except at the tips, and in having plicae rather than folds on the anterior tip of the tongue. The plicae of the tongue indicate affinities of Riolama with Ptychoglossus, Alopoglossus, and Ecpleopus. The former two have completely plicate tongues. Riolama differs from Ecpleopus by the much depressed toes, by the complete superciliary series, and by the longitudinal rather than diagonal rows of ventral scales. R. leucosticta is the only known species. Specimens of Euspondylus maculatus (the type species of the genus) that have been reported from Pacific coastal South America are apparently mislabelled. The species is reliably known from northern and central Amazonian Peru. Euspondylus stenolepis, on account of its completely plicate tongue and smooth scales on the forelimbs, is referred to the genus Ptychoglossus (new combination). Keys to Prionodactylus and Euspondylus based on these changes are included. The status of names proposed in or referred to Euspondylus and Prionodactylus is summarized. The genus Prionodactylus has as relatives Cercosaura, Pantodactylus, Aspidolaemus, and Pholidobolus. Cercosaura is distinct in scutellation. The other genera, particularly Aspidolaemus and Pantodactylus, may not be distinguishable from Prionodactylus. Hemipenial features of Pantodactylus and Cercosaura distinguish the individuals for which this organ was studied from the individuals of the other genera for which hemipenes were examined.