Most of the 78 species of amphibians and reptiles definitely known from
the rainforest in southern El Peten have extensive ranges in the
Atlantic lowlands of southern Mexico 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 Peten and northern
Alta Verapaz, whereas _Eleutherodactylus rostralis_ and _Thecadactylus
rapicaudus_ are at the northern and western limits of their
distributions in El Peten. Nine (11%) species have the center of their
distributions in El Peten and the Yucatan 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: