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
CiA(C)negas 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.
Two species of _Terrapene_ occur in the United States. _Terrapene
carolina_, having four recognized subspecies, has a nearly continuous
distribution from southern Maine, southern Michigan, and southern
Wisconsin, southward to Florida and the Gulf coast and westward to
southeastern Kansas, eastern Oklahoma and eastern Texas, and
characteristically inhabits wooded areas.
_Terrapene ornata_ is a characteristic inhabitant of the western
prairies of the United States, and ranges from western and southern
Illinois, Missouri, Oklahoma, and all but the extreme eastern part of
Texas, westward to southeastern Wyoming, eastern Colorado, eastern and
southern New Mexico, and southern Arizona, and, from southern South
Dakota and southern Wisconsin, southward to northern Mexico (Fig. 1).
It is the only species of the genus that occurs in both Mexico and the
United States. The northeasternmost populations of _T. ornata_,
occurring in small areas of prairie in Indiana and Illinois, seem to
be isolated from the main range of the species. The ranges of _T.
ornata_ and _T. carolina_ overlap in the broad belt of prairie-forest
ecotone in the central United States. Interspecific matings under
laboratory conditions are not uncommon and several verbal reports of
such matings under natural conditions have reached me. Nevertheless,
after examining many specimens of both species and all alleged
"hybrids" recorded in the literature, I find no convincing evidence
that hybridization occurs under natural conditions.
_Terrapene ornata_ differs from _T. carolina_ in having a low,
flattened carapace lacking a middorsal keel (carapace highly arched
and distinctly keeled in _carolina_), and in having four claws on the
hind foot (three or four in _carolina_), the claw of the first toe of
males being widened, thickened, and turned in (first toe not thus
modified in _carolina_). _Terrapene ornata_ is here considered to be
the most specialized member of the genus by virtue of its reduced
phalangeal formula, lightened, relatively loosely articulated shell,
reduced plastron, and lightly built skull, which completely lacks
quadratojugal bones (Fig. 2); most of these specializations seem to be
associated with adaptation for terrestrial existence in open habitats.
[Illustration: FIG. 1. Geographic distribution of _Terrapene
ornata_. Solid symbols indicate the known range of _T. o.
ornata_ and hollow symbols the known range of _T. o. luteola_.
Half-circles show the approximate range of intergradation
between the two subspecies. Triangles indicate localities
recorded in literature; specimens were examined from all other
localities shown. Only peripheral localities are shown on the
map.]
Two subspecies of _T. ornata_ an recognized. _Terrapene o. luteola_,
Smith and Ramsey (1952), ranges from northern Sonora (Guaymas) and
southern Arizona (southern Pima County) eastward to southeastern New
Mexico and Trans-Pecos, Texas, where it intergrades with _T. o.
ornata_; the latter subspecies is not yet known from Mexico but almost
surely occurs in the northeastern part of that country. The subspecies
_luteola_ differs from _ornata_ in being slightly larger and in having
more pale radiations on the shell (11 to 14 radiations on the second
lateral lamina in _luteola_, five to eight in _ornata_). In
individuals of _luteola_ the markings of the shell become less
distinct with advancing age and eventually are lost; shells of most
old individuals are uniform straw color or pale greenish-brown; this
change in coloration does not occur in _T. o. ornata_.
[Illustration: FIG. 2. Dorsal and lateral views of skull of
_T. o. ornata_ (_a_ and _b_) (KU 1172, male, from 6 ml. S.
Garnett, Anderson Co., Kansas) and of _T. carolina_
(_c_ and _d_)(KU 39742, from northern Florida). Note the
relatively higher brain-case and the incomplete zygomatic
arch in _T. o. ornata_. All figures natural size.]