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Two distinct populations of Leposoma occur in the Guianan highlands area of northern South America. Leposoma percarinatum Miiller is distinguished from L. guianense Ruibal by a higher number of transverse rows of dorsal scales (36-39, mean 37.5 in L. percarinatum; 31-35, mean 32.6 in L. guianense) as well as by more transverse rows of ventral scales and more scales around the midbody region. In L. percarinatum, the third and especially the fourth pairs of chin shields are much reduced compared to the conditions in L. guianense; the scale behind the longest supralabial is as small as the temporal scales, rather than larger as in L. guianense. L. percarinatum usually has a light line from the hindlimb insertion along the tail; this is absent in L. guianense. The interparietal scale in L. percarinatum is much larger than in L. guianense, but the head is relatively narrower. The distinctive long middle preanal scale noted by Ruibal is not a constant feature of L. guianense. All 30 specimens of L. percarinatum are female. We have examined 17 specimens of L. guianense; seven are male, 10, female. L. percarinatum is probably a unisexual taxon. Male specimens reported are probably L. guianense. In coloration, L. percarinatum seems to be intermediate between L. guianense and L. parietale, a species found on the Amazonian slopes of Peru, Ecuador and Colombia. In scale counts, size of fourth pair of chin shields, and head width, L. parietale and L. guianense are alike. The interparietals of L. percarinatum and L. parietale are both large. Geographically and morphologically, it seems likely that L. parietale and L. guianense hybridized to give rise to L. percarinatum. If this is so, these bisexual forms are probably distinct species, although the distributions and general similarities suggest that they represent subspecies of a single species. Leposoma ioanna, new species, is closely related to L. parietale. The two known specimens come from near Buenaventura, Valle del Cauca, on the Pacific coast of Colombia. L. ioanna is distinguished from L. parietale by having the third pair of chin shields separated from the infralabials by two small scales, and by a much reduced fourth pair. It may have a larger body size and lower mean numbers of dorsal scale rows, transverse ventral scale rows, and scales around midbody region. Two Chocoan specimens reported previously as Alopoglossus copii are Leposoma southi. Both L. ioanna and L. southi are species of Leposoma from the Pacific coast of Colombia, whence no species has been reported previously. L. southi also occurs in Costa Rica and Panamá east as well as west of the Canal Zone. Hemipenes of Leposoma have two characteristic rows of calcareous spines in each half. Features of the hemipenis of L. parietale are also seen in the hemipenis of Arthrosaura kockii, supporting the relation of the genera suggested on other bases. Evidence of past hybridization of L. parietale with L. guianense (the unisexual L. percarinatum) and the disjunct L. ioanna, which is related to L. parietale, indicate considerable changing of distributions of the genus Leposoma. Dates of changes in distribution are unknown.