[Illustration: FIG. 7. Pattern on the dorsal surface of the distal part of the right hind limb of some American species and subspecies of the genus _Trionyx_. Note the gradual reduction in contrast of pattern from that of _hartwegi_ (a) to that of _emoryi_ (d).
a. _T. spinifer hartwegi_ (KU 15932, × 3/4) b. _T. spinifer pallidus_ (KU 40175, × 2/3) c. _T. spinifer guadalupensis_ (TU 10165, × 3/4) d. _T. spinifer emoryi_ (KU 3153, × 5/6) e. _T. muticus muticus_ (KU 48228, × 3/4) f. _T. ferox_ (UMMZ 102276, × 1/2) ]
[Illustration: FIG. 8. Shape of tubercles on anterior edge of carapace in some American species and subspecies of the genus _Trionyx_ (× 1/2). Note the gradual reduction in size of tubercles from that of _hartwegi_ (b) to that of _muticus_ (h).
a. _T. ferox_ (UMMZ 90010) b. _T. spinifer hartwegi_ (KU 3346) c. _T. spinifer pallidus_ (TU 13213) d. _T. spinifer guadalupensis_ (TU 10160) e. _T. spinifer emoryi_ (KU 2906) f. _T. ater_ (KU 46906) g. _T. muticus muticus_ (KU 48229) h. _T. muticus muticus_ (KU 48232) ]
A corresponding sequence of change occurs in the size of dark markings on the dorsal surface of the limbs (Fig. 7). The hind limb usually has larger markings than the forelimb. The change is gradual from larger and darker markings (contrasting pattern) in _hartwegi_, _spinifer_ and _asper_ to smaller and paler markings (non-contrasting pattern) in _emoryi_.
There is also a cline in tuberculation (Fig. 8) that parallels geographically the sequence of changes in patterns mentioned immediately above. The size of the tubercles along the anterior edge of the carapace changes in both sexes from those that are enlarged and equilateral or conical in shape in _spinifer_, _hartwegi_, _asper_ and _pallidus_ to those that are scarcely elevated in _guadalupensis_, _emoryi_ and _T. ater_. Indeed, in the three kinds mentioned last, the tubercles are absent in some specimens. There seems to be a corresponding reduction in the size and number of small, sharp-tipped tubercles that cover the carapace in adult males; the carapace of _T. ater_ is mostly smooth and has only a few small, whitish tubercles.
[Illustration: FIG. 9. Anteroposterior position of plane of greatest width of carapace (CL/PCW) in some American species and subspecies of the genus _Trionyx_. Symbols as in Fig. 3. The greatest width of carapace is midway between anterior and posterior ends in _ferox_, _spinifer_, _hartwegi_, _asper_ and _muticus_, and farther posterior in the other subspecies of _spinifer_.]