I have not studied in detail the many fossil remains but such examination as I have made of them suggests that many of the characters used as a basis for distinguishing fossil forms in North America are subject to individual variation or are of no diagnostic value in the living species (Hummel, 1929:769). Knowledge of the variation in the living species of the Old World would aid in adequately appraising the North American fossils. Some osteological characters of the three living American species (excluding _ater_) together with data on variation within a given species are mentioned below. Some differences in skulls of the three species already were mentioned in the section "Osteological Characters." Because most fossil remains are those of the carapace and plastron, attention is here given to those structures. _Widened alveolar surfaces of jaws._--An ontogenetic variation affecting large skulls of _T. ferox_ and some individuals of _T. spinifer asper_; presumably confined to females. Of especial interest is its presence in some populations of _asper_ that are not otherwise distinguishable (external characters) from the rest of the individuals comprising that subspecies. _Sculpturing._--No differences in pattern (generally of anastomosing ridges) on carapace or plastron; fineness or coarseness seemingly correlated with size; frequency and kind (knoblike or ridgelike) of bony prominences on carapace variable; bony prominences confined to species _spinifer_ and _ferox_, occurring principally on large females. _Fontanelles of carapace._--Closure more or less correlated with increasing size, although much variation noted between individuals of same size; small individuals have fontanelles confluent (medially), thus separating nuchal from contact with first neural and first pair of pleurals. _Number and arrangement of neurals and pleurals._--Neurals number six to nine, usually seven or eight; pleurals number seven or eight pairs, and may or may not be in contact with each other posteriorly; eighth pair of pleurals when present reduced, never contacting seventh neural; arrangement posteriorly variable (see Fig. 16 and Tab. 5). _Plastral callosities._--Increase in size with advancing age causing corresponding reduction in size of plastral vacuity; relatively best developed in _muticus_ (all elements touching medially on KU 41380 leaving no plastral vacuity); probably no callosities on preplastra or epiplastron of _ferox_; callosity on epiplastron of _spinifer_ not covering entire surface (as it may in _muticus_). _Epiplastron._--Obtusely-angled (greater than 90 degrees) in _muticus_; acutely-angled (90 degrees or less) in _ferox_ and _spinifer_. _Hyo-hypoplastral suture._--Usually present, but occasionally absent, in all species.