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.
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 [459] 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.