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) ] _Pattern on Dorsal Surface of Limbs_ 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_. _Tuberculation_ 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_.] _Ratios_ The clinal tendencies in PL/HW (Fig. 3) and CL/CW (Fig. 4) that parallel those mentioned above for pattern and tuberculation have already been mentioned under the section "Ontogenetic Variation." The ratio of CL/PCW (Fig. 9) was used in an effort to show further differences in the shape of the carapace, especially the plane on the carapace where the greatest width occurs. Figure 9 shows the greatest width to be approximately midway between the anterior and posterior ends in the subspecies _spinifer_, _hartwegi_ and _asper_, and in the species _ferox_ and _muticus_ (CL/PCW of 2.00). The greatest width of carapace is more posterior and at approximately the same plane in _pallidus_ and _guadalupensis_, and farther posterior in _emoryi_. Calculated ratios for 12 specimens of _T. ater_ average 2.15, a value that suggests closer affinity with _pallidus_, _guadalupensis_ and _emoryi_ than to the other species and subspecies.