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The name Enicognathus melanauchen Jan belongs to a currently unrecognized but apparently valid species of small snake described some 140 years ago from an unknown locality in Bahia, Brazil; the unique specimen is in the Nationaal Natuurhistorisch Museum, Leiden. Attempts were unsuccessful to associate it with named genera of the colubrid subfamilies Xenodontinae, Dipsadinae, Colubrinae, and Natricinae; consequently, no basis was found for challenging the provenance. The genus Amnesteophis and tribe Amnesteophiini therefore are erected for the species. (The snake generic name Enicognathus and the emendation Henicognathus are both junior homonyms of bird names.) Assignment to Xenodontinae is partly on geographic grounds as well as on hemipenial and osteological comparisons with the other colubrid subfamilies mentioned.
Tribe Amnesteophiini differs from other tribes in Xenodontinae by combination of three unusual character states: (1) straight-line configuration of posterior maxillary teeth; (2) lack of hemipenial calyces; and (3) an unforked sulcus spermaticus. Hemipenial calyces also are absent in the Xenodontini, and an unforked sulcus spermaticus occurs in one species of Echinantherini (Taeniophallus nicagus). Amnesteophis melanauchen somewhat resembles Taeniophallus occipitalis in color pattern and 15 dorsal scale rows, but A. melanauchen differs externally from all Taeniophallus in having feebly keeled dorsal scales. The provenance and taxonomic placement of Amnesteophis needs to be corroborated by new specimens, or refuted by new insight on relationships and geographic origin.
Comparisons made for this study shed light on a few groups of dipsadines. The tribal name “Leptodeirini Jenner” auctorum is nomenclaturally unavailable and invalid since it was proposed in an unpublished thesis (Jenner, 1981) that cannot subsequently be used as an indication or bibliographic reference in order to confer availability (contra Dowling et al., 1983). The diagnostic character of a simple sulcus spermaticus does not strictly characterize the type genus Leptodeira as originally assumed. Several molecular studies have failed to corroborate monophyly for the group of genera originally and subsequently assigned to the “Leptodeirini,” although two genera (ImantodesLeptodeira) consistently received strong support. The biogeographically and ecologically successful Imantodes and Leptodeira, which share large geographic ranges between the tropics of Cancer and Capricorn, are placed in the new tribe Imantodini. In addition to molecular support, the loss of bifurcation of the sulcus spermaticus (Imantodes and some Leptodeira) or reduction to a small terminal fork or expanded flat area (other Leptodeira) sets off the mainly scansorial Imantodini from all other Dipsadinae except the terrestrial Mesoamerican-North American night-snakes, which appear to form an unnamed clade.
The dipsadine genus Rhadinaea Cope (sensu Myers, 1974) is further partitioned by the resurrection of RhadinellaSmith, 1941, which equates to the godmani species group of 15 Middle American species. The maxillary dentition in Rhadinella is superficially similar to that of Amnesteophis in the straight-line arrangement of several posteriorly enlarged teeth (ultimate fang not offset). Similar apomorphic dentition is shared with Lower Central American Trimetopon, which differs from Rhadinella in tendency toward diminutiveness and lack of a basal nude pocket on the hemipenis. Offset fangs (whether grooved or smooth) appear symplesiomorphic within most genera of