[Illustration: FIG. 5. Pattern on dorsal surface of snout of some American species and subspecies of the genus _Trionyx_. Note the gradual transition in pattern from that of _hartwegi_ (b) and _asper_ (c) to that of _emoryi_ (h). a. _T. ferox_ (UMMZ 102276, × 1/3) b. _T. spinifer hartwegi_ (KU 46742, × 3/4) c. _T. spinifer asper_ (KU 50842, × 1) d. _T. spinifer pallidus_ (KU 2958, × 1/2) e. _T. spinifer pallidus_ (KU 2934, × 1/2) f. _T. spinifer pallidus_ (KU 2947, × 1/2) g. _T. spinifer guadalupensis_ (TU 10165, × 2/3) h. _T. spinifer emoryi_ (KU 48218, × 2/3) i. _T. muticus muticus_ (KU 48236, × 2/3) ] Geographic Variation Geographic variation occurs in _Trionyx spinifer_ and _T. muticus_. The variant populations of _spinifer_ are segregated into six subspecies, those of _muticus_ into two. In the subspecies of _spinifer_ there is both group variation and clinal variation. Group Variation The six subspecies of _spinifer_ can be separated into two groups on the basis of the juvenal pattern. One group (subspecies _spinifer_, _hartwegi_ and _asper_) has a pattern of dark spots or ocelli of various sizes on the carapace, whereas the other group (subspecies _pallidus_, _guadalupensis_ and _emoryi_) has a pattern of small white dots or tubercles on the carapace. The two groups differ also in the manner in which the mottled and blotched pattern first appears on the carapace of females. Usually, contrasting lichenlike figures initially surround the dark spots or ocelli on the carapace in females of the _spinifer_ group (less evident in _pallidus_), whereas females of the _emoryi_ group usually lack a contrasting pattern early in ontogeny. In general, the two groups differ in the degree of pigmentation. The _spinifer_ group has larger marks and more contrasting patterns on the head and limbs, and more extensive pigmentation on the ventral surface than members of the _emoryi_ group. _T. ater_ is more closely related to those subspecies of the _emoryi_ group but differs in having the ventral surface heavily speckled with black and an over-all blackish, dorsal coloration; the underlying pattern of _ater_ resembles that of _emoryi_. Clinal Variation Several characters are arranged in a geographical gradient or cline. Some characters are relatively uniform and represent a terminus in the _spinifer_ group. Some characters change gradually and successively through the subspecies _pallidus_ and _guadalupensis_, and terminate in _emoryi_ and _T. ater_. Some characters of _ater_, in turn, show affinity with _T. muticus_ and _T. ferox_. _Pattern on Snout_