Pattern on Carapace
Two features of the pattern on the carapace are of taxonomic worth: 1) the width and distinctness of the pale rim at the periphery of the carapace (marginal rim), if present, and 2) the kind of pattern on the carapace (juvenal pattern). The marginal rim is absent in females of T. ater, and only faintly evident in males. The marginal rim is obscured or absent (adult males and females) and is not separated from the ground color of the carapace by a dark marginal line in hatchlings of T. ferox. The carapace of T. muticus has a marginal rim that is usually separated from the ground color of the carapace by an ill-defined, dark marginal line; some individuals lack the marginal dark line. The subspecies of T. spinifer have a well-defined, dark, marginal line that separates the marginal rim from the ground color of the carapace; T. s. asper has more than one dark marginal line on the carapace. The marginal rim is ill-defined and blotched, or absent, in large females of all species of Trionyx.
The marginal rim is widest at the posterior end of the carapace and lacking in the nuchal area. The width of the pale marginal rim is very narrow, almost to the degree of being absent, in juveniles of T. ferox. T. s. emoryi has a pale, marginal rim that is four or five times wider posteriorly than it is laterally, whereas posteriorly the width of the rim in the other subspecies of T. spinifer and in the species T. muticus is only two or three times wider posteriorly than it is laterally.
The juvenal pattern commonly consists of whitish tubercles or dots (T. s. emoryi, T. s. guadalupensis, T. s. pallidus, T. ater), large black ocelli (T. s. spinifer), small black dots and ocelli (T. s. hartwegi, T. s. asper), large dusky spots or ocelli (T. m. calvatus), or small dusky dots or short streaks and dashes (T. m. muticus). Some hatchlings of pallidus and emoryi have a uniform pale brown or tan carapace; hatchlings of T. ferox have a distinctive pattern ([Pl. 31]). [463] Further comments and illustrations pertaining to kind of pattern on the carapace are offered under the accounts of species and subspecies.
Pattern on Dorsal Surface of Snout ([Fig. 5])
T. ferox has pale stripes on a dark background that unite in front of the eyes; the dark ground color becomes paler with increasing size, but the stripes retain thick black borders. T. m. muticus has ill-defined, pale stripes that are evident just in front of the eyes and do not extend anteriorly to unite in front of the eyes, whereas T. m. calvatus lacks pale stripes on the snout. The kind of pattern on the dorsal surface of the snout that is characteristic for each of the subspecies of T. spinifer has been mentioned in the discussion of clinal variation.
Pattern on Side of Head ([Fig. 6])
T. ferox has a pale broad, postocular stripe in contact with the orbit or not, and other pale marks on a dark background; the ground color becomes paler with increasing size, but the stripes and other marks retain thick black borders. T. m. muticus usually has an uninterrupted, dusky-bordered, postocular stripe, whereas T. m. calvatus (in adult males only) has pale postocular stripes with thick blackish borders. The pattern on the side of head that is characteristic for each subspecies of T. spinifer has been mentioned in the discussion of clinal variation.
Pattern on Dorsal Surface of Limbs ([Fig. 7])
Young specimens of T. ferox have pale marks on a blackish background. As growth proceeds the distinctive contrasting pattern is obliterated and eventually is replaced by a uniform grayish coloration in large adults. The pattern on the limbs of T. muticus is not contrasting, and is almost a uniform grayish, consisting of fine, pale markings. The clinal variation in pattern and kind of pattern on the limbs of the subspecies of T. spinifer has been mentioned in the discussion of clinal variation. Dark markings tend to form streaks that are coincident with the digits, and larger markings occur on the hind limbs than on the forelimbs.