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_.