I am grateful also to the Kansas Academy of Science for three research grants (totaling $175.00) that supported part of the work. The brief discussion of taxonomic relationships and distribution results partly from studies made by means of two research grants (totaling $150.00), from the Graduate School, University of Kansas, for which I thank Dean John H. Nelson.
Systematic Relationships and Distribution
Turtles of the genus _Terrapene_ belong to the Emyidae, a family comprising chiefly aquatic and semiaquatic species. _Terrapene_, nevertheless, is adapted for terrestrial existence and differs from all other North American emyids in having a hinged and movable plastron and a down-turned (although often notched) maxillary beak. _Emydoidea blandingi_, the only other North American emyid with a hinged plastron, lacks a down-turned beak. The adaptations of box turtles to terrestrial existence (reduction of webbing between toes, reduction in number of phalanges, reduction of zygomatic arch, and heightening of shell) occur in far greater degree in true land tortoises of the family Testudinidae. Four genera of emyid turtles in the eastern hemisphere (_Cuora_, _Cyclemys_, _Emys_, and _Notochelys_) possess terrestrial adaptations paralleling those of _Terrapene_ but (with the possible exception of _Cuora_) the adaptations are less pronounced than in _Terrapene_. A movable plastron has occurred independently in two groups of emyids in the New World and in at least three groups in the Old World.
The genus _Terrapene_, in my view, contains seven species, comprising 11 named kinds. Of these species, five are poorly known and occur only in Mexico. _Terrapene mexicana_ (northeastern Mexico) and _T. yucatana_ (Yucatan peninsula) although closely related, differ from each other in a number of characters. Similarly, _Terrapene klauberi_ (southern Sonora) and _T. nelsoni_ (Tepic, Nayarit--known from a single adult male) are closely related but are considered distinct because of their morphological differences and widely separated known ranges. _Terrapene coahuila_, so far found only in the basin of Cuatro Ciénegas in central Coahuila, is the most primitive _Terrapene_ known; it differs from other box turtles in a number of morphological characters and is the only member of the genus that is chiefly aquatic.