Eutrachelophis, new genus is established to accommodate E. bassleri, new species, and E. steinbachi (Boulenger), new combination; a third species close to E. bassleri awaits naming. These taxa are placed in the Eutrachelophiini, new tribe, to express hypothesized relationship with the Xenodontini, which are defined by presence of hemipenial apical discs (a character lost in several species). The acalyculate spiny hemipenis of Eutrachelophis bassleri is unique among “xenodontines” in having a noncapitate, well-formed capitulum in the form of a nude dome; bifurcation is lacking even in the insertion of the major retractor muscle; the sulcus spermaticus is centrolineal in the retracted organ but becomes centrifugal during eversion.

The hemipenis of Eutrachelophis steinbachi is strikingly different in being deeply divided, with long spiny lobes tipped with tufts of sender spines, but it resembles those of some other colubrids (e.g., South American Xenodon suspectus; African Mehelya poensis). Based on hemipenial comparisons, E. bassleri and E. steinbachi seem unlikely congeners. Nonetheless, global comparisons of viscera, head glands, head muscles, color pattern, skull, and dentition indicate that they are congeneric despite hemipenial differences. Neither E. bassleri nor E. steinbachi shows sufficient resemblance to any other “xenodontine” that would suggest an alternative phylogeny. Overall resemblance in so many details, especially of the skull, is not reasonably explained by convergence.

Therefore, contrary to dogma, the hemipenes in this case provide no clues to generic affinity. An explanatory hypothesis has Eutrachelophis bassleri and E. steinbachi derived from common stock, but with hemipenial lobes in the bassleri lineage suppressed during embryonic development. It further suggests that the unusual broad, hemispherical nude apex in E. bassleri is homologous with the interlobular smooth, expandable terminal basin in E. steinbachi. The hemipenial differences in Eutrachelophis are not inconsistent with growing awareness that evolution of male genitalia may outpace changes in other characters without predictable limits to complexity. Fine-scale Hox gene expression might account for the novel hemipenis of E. bassleri.

Although it is well established that snake hemipenes generally give at least a hint of relationship, a widely held belief that they are taxonomically stable and relatively free of selection pressures must be abandoned. Hemipenes (and probably female cloacae) are not “neutral” or “uncorrelated” characters but are subject to intense selection pressure requiring successful copulation, hence successful reproduction. The belief that one description or illustration suffices to typify a species (or genus) has no merit without proper sampling. Intraspecific variation is commonplace in geographically widespread species—sometimes, not always, signaling the presence of unnamed cryptic species.

Examples are given of intraspecific variation in different kinds of hemipenial features. Also provided are examples of evolutionary plasticity and extreme divergence in snake hemipenes, with a few references to female cloacae, about which much less is known. The hemipenes of two apparent sister species, Enulius flavitorques and “Enuliophis” sclateri, might represent a case of hemipenial divergence as extreme as seen between Eutrachelophis bassleri and E. steinbachi. Attention is called to examples of extraordinary folding and coiling of retractor muscles and even a folding hemipenis, all of which enable long hemipenes to fit within short tails in both Scolecophidia (Typhlina) and Alethinophidia (Prosymna). Folding of the hemipenis and retractor muscle