The closable shell of box turtles is of obvious survival value in providing protection for the soft parts of the body. In most of the species of _Terrapene_, the lobes of the plastron completely close the openings of the shell; closure is so tightly effected in some individuals that it is difficult to insert the blade of a knife between the adpressed margins of carapace and plastron. In _T. ornata_ nevertheless, both lobes of the plastron are deficient on their lateral margins; four narrow openings remain when the lobes are drawn shut. Emargination of the plastron has occurred at the places where the limbs rub against it during locomotion. This reduction of the plastron permits the body to be held off the ground during forward locomotion and seemingly permits a generally freer range of movement for the limbs. The possible disadvantages of an imperfectly closable shell seem to be compensated for by increased mobility. Reduction of the plastron is correlated with a general lightening of the shell, probably associated with the increased vagility of this species. Lightening of the shell is evident also in the relatively thin, loosely articulated bony elements. Shells of adult _T. ornata_ that are old and weathered, or macerated (unless they are partly co-ossified because of injury), can nearly always be disarticulated with ease, whereas the bony elements in the shells of adult _T. carolina_ (all races) are nearly always co-ossified or separable only after prolonged maceration. The relatively low, flattened shell of _T. ornata_ is an adaptation associated with the tendency to seek shelter in the limited space of earthen forms, burrows, or small natural cavities in the course of the warm season and to burrow more deeply into the ground in winter. _Terrapene ornata_ is, in fact, the only species of the genus that may be considered an habitual burrower. Individuals of _T. carolina_ tend to seek shelter in the warm season by making forms in dense vegetation or by digging into yielding substrata such as mud or humus, although they may burrow deeply into the earth in winter. Extreme weakness or absence of the middorsal keel of _T. ornata_ seems to be a modification associated with burrowing habits and general adaptation to terrestrial life; the keel is similarly reduced in testudinids. Retention of epidermal laminae (as opposed to regular exfoliation of the older parts of scutes) occurs in all box turtles, in several other groups of terrestrial emyids, and in testudinids. The phenomenon is here considered to be a specialization of scute shedding--developed in terrestrial and semiterrestrial chelonians--that provides additional protection to the shell against wear and minor injuries.