habitat includes permanent streams and rivers (Pl. 10, fig. 1), some of which are clear and others muddy. In the rainy season temporary ponds form in depressions on the forest floor (Pl. 10, fig. 2); these are important as breeding sites for many species of amphibians. Aquatic members of the herpetofauna are here considered to be those species that either spend the greatest part of their lives in the water or usually retreat to water for shelter. Seven species of turtles and one crocodilian are aquatic. Of these, Dermatemys mawi, Staurotypus triporcatus, and Pseudemys scripta ornata inhabit clear water, whereas Chelydra rossignoni, Claudius angustatus, Kinosternon acutum, and K. leucostomum inhabit muddy water. Crocodylus moreleti apparently inhabits both clear and muddy water, for in the dry season it lives along the clear rivers, but in the rainy season inhabits flooded areas in the forest as well.

2. Aquatic Margin.—Extensive marshes were lacking in the part of southern El Petén that I visited; consequently, the aquatic margin habitat is there limited to the edges of rivers and borders of temporary ponds. Bufo marinus, Rana palmipes, and Rana pipiens are characteristic inhabitants of the aquatic margin, although in the rainy reason Bufo marinus often is found away from water. Observations indicate that Tretanorhinus nigroluteus lateralis inhabits the margins of ponds and streams and actually spends considerable time in the water. Although Iguana iguana rhinolopha is arboreal, it lives in trees along rivers, into which it plunges upon being disturbed. Species included in this category are those that customarily spend most of their lives at the edge of permanent water. Frogs and toads that migrate to the water for breeding and the snakes that prey on the frogs at that time are not assigned to the aquatic-margin habitat.

3. Fossorial.—Characteristic inhabitants of the mulch on the forest floor are Bolitoglossa moreleti mulleri, Lepidophyma flavimaculatum flavimaculatum, Scincella cherriei cherriei, Ninia sebae sebae, Pliocercus euryzonus aequalis, and Micrurus affinis apiatus. Other species of snakes that spend most of their lives above ground often forage in the mulch layer; among these are Coniophanes bipunctatus biserialis, Coniophanes fissidens fissidens, Coniophanes imperialis clavatus, Lampropeltis doliata polyzona, and Stenorrhina degenhardti. Among the amphibians, at least Hypopachus cuneus nigroreticulatus, Eleutherodactylus rostralis, and Syrrhophus leprus are known to seek shelter in the mulch.

4. Terrestrial.—One turtle, Geoemyda areolata, is primarily terrestrial. Among the lizards, conspicuous terrestrial species are Anolis humilis uniformis and Ameiva festiva edwardsi; Anolis lemurinus bourgeaei and Basiliscus vittatus spend part of their lives on the ground, but also live on trees and in bushes. Eumeces schwartzei and E. sumichrasti apparently are terrestrial. The only terrestrial lizard that is nocturnal is Coleonyx elegans elegans, which by day hides in the leaf litter or below ground. Nocturnal amphibians that are terrestrial include Bufo marinus, Bufo valliceps valliceps, Eleutherodactylus rugulosus rugulosus, Syrrhophus leprus, and Hypopachus cuneus nigroreticulatus. A large number of active diurnal snakes are terrestrial; these include Boa constrictor imperator, Clelia clelia clelia, Dryadophis melanolomus laevis, Drymarchon corais melanurus, Drymobius margaritiferus margaritiferus, Pseustes poecilonotus poecilonotus, and Spilotes pullatus mexicanus. Nocturnal terrestrial snakes include three kinds of Bothrops (B. atrox asper, B. nasutus, and B. nummifer nummifer), all of which seem to be equally active by day.

5. Arboreal.—In this habitat the third dimension (height) of the rainforest probably is the most complex insofar as the inter-relationships of species and ecological niches are concerned. I have attempted to categorize species as to microhabitats within the arboreal habitat; in so doing, I recognize four subdivisions—bushes, tree trunks, tree tops, and epiphytes.

Bush inhabitants include several species of lizards and snakes, all of which have rather elongate, slender bodies, and long tails. Common bush-inhabitants in southern El Petén are Anolis limifrons rodriguezi, Basiliscus vittatus, Laemanctus deborrei, Leptophis mexicanus mexicanus, and Oxybelis aeneus aeneus. All of these are diurnal, and all but Laemanctus have been observed sleeping on bushes at night.

Tree-trunk inhabitants include five species of lizards. Thecadactylus rapicaudus lives on the trunks of large trees; Sphaerodactylus lineolatus lives beneath the bark on dead trees and on corozo palms. Anolis lemurinus bourgeaei lives on the bases and buttresses of large trees, from which it often descends to the ground. Corythophanes cristatus and Anolis capito were found only on tree trunks and large vines.

The least information is available for the species living in the tree tops. The following species were obtained from tops of trees when they were felled, or have been observed living in the tree tops: Anolis biporcatus, Iguana iguana rhinolopha, Celestus rozellae, Leptodeira septentrionalis polysticta, Leptophis ahaetulla praestans, Sibon dimidiata dimidiata, and Sibon nebulata nebulata.

Epiphytes, especially the bromeliads, provide refuge for a variety of tree frogs and small snakes. Of the tree frogs, Hyla picta, Hyla staufferi, Phyllomedusa callidryas taylori, Similisca baudini, and Similisca phaeota cyanosticta have been found in bromeliads; other species probably occur there. Among the snakes, Imantodes cenchoa leucomelas, Leptodeira frenata malleisi, Leptodeira septentrionalis polysticta, Sibon dimidiata dimidiata, and Sibon nebulata nebulata are frequent inhabitants of bromeliads; all of these snakes are nocturnal.

Relationships of the Fauna

Most of the 78 species of amphibians and reptiles definitely known from the rainforest in southern El Petén have extensive ranges in the Atlantic lowlands of southern México and Central America; many extend into South America. Sixty-two (80%) of the species belong to this group having extensive ranges in Middle America. Three species (Syrrhophus leprus, Leptodeira frenata, and Kinosternon acutum) are at the southern limits of their distributions in southern El Petén and northern Alta Verapaz, whereas Eleutherodactylus rostralis and Thecadactylus rapicaudus are at the northern and western limits of their distributions in El Petén. Nine (11%) species have the center of their distributions in El Petén and the Yucatán Peninsula; representatives of this group include Claudius angustatus, Dermatemys mawi, Laemanctus deborrei, and Eumeces schwartzei.

In determining a measure of faunal resemblance, I have departed from the formulae discussed by Simpson (1960) and have analyzed the degree of resemblance by the following formula used to calculate an index of faunal relationships: