We were not surprised when a small snake was recovered from the stomach of a M. nigrocinctus from southern Mexico. Following the key to Mexican genera of snakes in Smith and Taylor (1945), this snake is identified as Geophis, which—on the basis of a suite of characters—it is not. Subsequent examination clearly indicated that this prey item represented a previously unknown snake, unlike anything currently recognized from the New World tropics. This small snake was obtained now over 40 yr ago, and the report of its discovery has been a long time in coming. We were optimistic that additional specimens might be secured, but after at least a dozen more trips into the region spanning several decades, we have been unrewarded.
Type Species.—Cenaspis aenigma, new species (described below). Diagnosis of Genus.—This genus differs from all other New World dipsadids by two unique features: 1) all subcaudals undivided and 2) hemipenis single, noncapitate, hemipenial body and apical region completely covered with calyces, and sulcus spermaticus simple.