PROC. BIOL. SOC. WASH. 102(4), 1989, pp. 1045-1049 THE KARYOTYPE OF EXILIBOA PLACATA BOGERT (TROPIDOPHEIDAE), AND COMPARISONS WITH THE FAMILY BOIDAE (REPTILIA: SERPENTES) Laurence M. Hardy Abstract.— The first karyotype for any member of the Tropidopheidae is described from one male and one female of the dwarf boa, Exiliboa placata Bogert, from Oaxaca. The diploid number is 36, composed of 16 macrochro-mosomes and 20 microchromosomes. A possible secondary constriction is present on the second pair of macrochromosomes, and this constitutes the only distinctive difference between this species and some members of the Boidae. The significance of possible differences in centromere positions between Exi-liboa and boids with 36 chromosomes is unknown. The Tropidopheidae includes four genera according to several recent authors (Under-wood 1976, McDowell 1987): Tropidophis (15 species), Trachyboa (2 species), Unga-liophis (2 species), and Exiliboa (monotyp-ic). Nothing is known of the chromosome morphology for any member of the family. This paper reports the karyotype of the monotypic genus Exiliboa. Two specimens of Exiliboa placata Bo-gert were available for study: an adult fe-male (UTA R-4731) and an adult male (UTA R-4732). These specimens were col-lected by Jonathan A. Campbell at 7.1 mi. (UTA R-4731) and 6.5 mi. (UTA R-4732), respectively, north of the crest of Cerro Pe-16n, Oaxaca, Mexico, on 28 June 1975. Chromosomes were prepared by the hy-potonic citrate method of Patton (1967), us-ing the modification by Cole & Leavens (1971). Velban was used instead of colchi-cine. Each macrochromosome was mea-sured (to the closest 0.01 mm) with dial calipers directly on the 4x5" negative. Chromosome terminology follows Cole (1970). The arrangement of the chromo-somes within the karyotype is based on size, from the largest pair (number one) to the smallest. In addition to the karyotypes pre-sented (Figs. 1, 2), I subjected the measure-ments of the best 1 9 cells (seven cells from the female and twelve cells from the male) to computer analysis using the program Ka-rypak (ver. 1.0) by William H. LeGrande (pers. comm.). The macrochromosome means were calculated from each arm of each chromatid. In this analysis only mac-rochromosomes were measured and, for purposes of the karyotype percentages and arm ratio (centromeric index) estimations, they were treated as the entire complement (i.e., microchromosomes were not included as part of the karyotype). This process does not allow for the detection of differences among the microchromosomes nor for the contribution of the microchromosomes to the entire karyotype. However, for most snakes such information on the microchro-mosomes is rarely available and any differ-ences in size are suspect, in most cases, be-cause of the small sizes and poor resolution. Therefore, omission of the microchromo-somes is practically the same as assignment of a constant. I believe that this approach is most effective and reasonable for the crit-ical examination of the macrochromo-somes. Since no sexual dimorphism was de-tected, the male cells were combined with the female cells for the construction of the composite idiogram (Fig. 3).