The transition from the dark coloration of hatchlings to the paler coloration of adults is gradual and subject to individual variation. The loss of dark color ventrally occurs first on the plastral area, then the hind limbs, forelimbs, posterior part of carapace and last on the neck and throat. The soft parts of the body dorsally are gray or dark gray, and do not become so pale as the ventral surface. The smallest specimen that I have seen displaying the dark features of the hatchlings is a male, 7.7 centimeters (UMMZ 100673); a female, 9.5 centimeters (UMMZ 110987), is the smallest specimen having a whitish plastral area. The change from dark to pale coloration on the ventral surface occurs at a size of 8.0 to 9.0 centimeters. The largest specimens I have seen having indistinct, dusky blotches of the underside of the carapace are a female, 11.3 centimeters (UMMZ 100836), and a male, 16.0 centimeters (UMMZ 106322). A contrasting pattern on head and limbs, and a dark throat are still evident in a female 19.2 centimeters (UMMZ 106302). _Comparisons._--_Trionyx ferox_ can be distinguished from all other species of the genus in North America by the presence of a marginal ridge, longitudinal ridges of tubercles on the carapace of juveniles (less evident in adults), and the unique juvenal pattern and coloration. The lack of a juvenal pattern and a smooth surface on the carapace (not gritty like sandpaper) distinguish adult males from those of _T. spinifer_. Most adults of both sexes can be distinguished from _spinifer_ and _muticus_ by the extension of the plastron farther forward than the carapace (developed to a slight degree in some specimens of _T. s. emoryi_). Both sexes of all ages can be distinguished from _muticus_ by the presence of knoblike tubercles on the anterior edge of the carapace, and septal ridges. _T. ferox_ is the largest species in North America; the maximum size of the plastron in adult males is approximately 26.0 centimeters (16.0 in _spinifer_) and of adult females, 34.0 centimeters (31.0 in _spinifer_). The head is wider in _ferox_ than in _muticus_ and most subspecies of _spinifer_ (closely approached by _asper_, _guadalupensis_, _emoryi_ and _T. ater_). The carapace is narrower in _ferox_ than in _muticus_ and most subspecies of _spinifer_ (closely approached by _emoryi_ and _T. ater_). The snout is shortest in _ferox_, but almost as short in _T. s. emoryi_ and _T. ater_. _T. ferox_ has proportionately the longest plastron in relation to length of carapace.