Sibynomorphus, an assemblage of about a dozen species of South American gastropod-eating colubrids, has a peculiar distribution. Six species occur in northern Peru and southwestern Ecuador. The others are distributed south of the Amazon basin in Brazil, Bolivia, Paraguay, Uruguay, and Argentina. Species of Sibynomorphus known from Ecuador and Peru are reviewed. Sibynomorphus oligozonatus and S. petersi are reported from Peru for the first time. Study of existing collections considerably amplifies understanding of the geographic ranges and character variation within S. oligozonatus, S. oneilli, S. petersi, and S. williamsi. Sibynomorphus vagrans is known only from its type locality (Bellavista, Cajamarca Department, Peru), and S. vagus (type locality unknown) is known only from near the single historically reported locality for the species (Huancabamba, Piura Department, Peru). Sibynomorphus oligozonatus (four specimens known) is distributed in southwestern Ecuador (Azuay and Loja Provinces) and northern Peru (Piura Department). Sibynomorphus oneilli is distributed in the Cordillera Oriental and Cordillera Occidental of northern Peru from southern Ancash Department to southern Cajamarca and Amazonas Departments. Available specimens of S. petersi extend the known range from Azuay and Loja Provinces in southwestern Ecuador, along the western Andean slopes to southern Ancash Department, Peru (Pacific versant); a single specimen is also known from the upper Río Chotano in central Cajamarca Department, thus documenting that this species occurs on the Amazonian versant. Sibynomorphus williamsi is known from central Peru in Lima and Ancash Departments from near sea level to at least 2,900 m in the Andes (and perhaps as high as 3,600 m)—an elevational range unmatched in any other species of the genus. Two species (S. vagrans and S. vagus) are restricted to the Amazonian versant, two are restricted to the Pacific versant (S. oligozonatus and S. williamsi), and two are found on both versants (S. oneilli and S. petersi). A key to the species of Sibynomorphus in Peru and Ecuador is provided.

Hemipenes are described for Sibynomorphus oligozonatus, S. petersi, S. vagrans, S. vagus, and S. williamsi. Their organs are similar to hemipenes of other Dipsadini in having a well-defined capitulum ornamented with papillate calyces, several rows of large spines encircling the organ proximal to the capitulum, an exceptionally large basal nude pocket, and the division of the sulcus spermaticus within the capitulum. In Sibynomorphus and in some other Dipsadini the branches of the sulcus spermaticus are centrolineal, but their tips pass somewhat to the lateral surface of the hemipenial lobe(s). The exceptional size of the nude pocket might be a synapomorphy of Dipsadini but further comparative studies are needed, especially because the pocket has probably been overlooked in many species.

The natural history of all northern species of S