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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 is illustrated for the African Prosymna ambigua bocagii, which has hemipenes longer than the tail. Axial architecture and limblessness of snakes have been attributed to Hox gene expression, which we suggest may also be the mechanistic basis for the appearance of hemipenial novelties without gradual change. Myers (1986) and the late Garth Underwood (2001, letters published herein), working with biometrician Clive Moncrieff, had independently concluded that the apical disc—the defining character of tribe Xenodontini—has been lost in some populations of the geographically widespread Xenodon “rabdocephalus” (Wied). That finding is here extended also to Xenodon suspectus Cope, which is resurrected from the synonymy of X. “rabdocephalus” based on a variable but cohesive color pattern and a hemipenis tipped with slender apical spines. A lectotype is designated for X. rabdocephalus, sensu stricto, which has been said to lack apical discs; the lectotype, however, has small apical discs on long slender lobes. Underwood and Moncrieff did not find the apical disc on hemipenes in Central American or in most South American populations of Xenodon “rabdocephalus.” Its absence is confirmed for populations in Central America and northwestern Colombia, for which the name Xenodon angustirostris W. Peters, 1864 (type locality Veragua) is tentatively resurrected. But sample sizes are small and hemipenial characteristics need to be elucidated for more populations. In addition to unrecognized species, South American “rabdocephalus” overall includes X. angustirostris, X. suspectus, and X. rabdocephalus sensu stricto. Xenodon rabdocephalus sensu lato is a complex of an unknown number of cryptic or “hidden” species, in which speciation events appear to be signaled by hemipenial changes. However, hemipenial data are too sparse at this time to allow separation of taxa because color patterns maintain a degree of consistency throughout an enormous geographic range (Mexico to Bolivia). These fer–de-lance mimics presumably are under strong selection pressure to maintain a Bothrops-like color pattern. A comprehensive study is needed, with molecular input to the extent possible. Loss of the apical disc among species of Xenodon may have been confined to species like “rabdocephalus” that have long-lobed hemipenes primitively tipped with small discs. Such loss characters can helpfully define species, but do not suffice for genera. Junior synonyms of Xenodon include Acanthophallus Cope, 1894 (type species Xenodon colubrinus Günther, 1858); and Thalesius Yuki, 1993 (type species Xenodon werneri Eiselt, 1963), both based on absence (loss) of the apical disc. Waglerophis Romano and Hoge, 1972, with apical discs on long hemipenial lobes, also is a synonym. (These interpretations of Xenodon synonymy agree with Zaher, 1999, and Zaher et al., 2009.) Terminology applicable to snake hemipenes is reviewed, particularly for a few characters needing clarification. Illustrations show the difference between capitation and noncapitation of hemipenes or hemipenial lobes with distinct capitula (“heads”). A commonly overlooked character—the smooth terminal basin of some bilobed hemipenes is described. The simple tripartite system for describing the orientation of sulci spermatici is updated. Complementary comparisons of retracted and everted hemipenes are encouraged, to provide better understanding and sometimes to increase the number of taxonomically useful characters. A discussion is appended of the Vidian canal and venous foramina in the prootic of Alethinophidian snakes. The Vidian canal in Pythonidae and the fossil Dinilysia shows the most lizardlike, presumably primitive, pattern. Anilioids show departures from this pattern, as do Acrochordoidea and Colubroidea. Boidae differ from Pythonidae in the loss of a true basipterygoid process (replaced, at least embryonically, by a cartilaginous nodule). Among the variable colubroid conditions, the Vidian canal in Eutrachelophis bassleri and E. steinbachi is similar to that of Liophis in being short and entirely enclosed in the sphenoid, with its anterior orifice set well in from the sphenoid-parietal suture.

New Taxa and Cryptic Species of Neotropical Snakes (Xenodontinae), with Commentary on Hemipenes as Generic and Specific Characters