The pattern (Fig. 5) on the snout usually consists of pale, dark-bordered stripes that form an acute angle in front of the eyes in _spinifer_, _hartwegi_ and _asper_, but the corresponding marks form a dark triangle the base line of which joins the anterior margins of the orbits in _emoryi_ and usually in _guadalupensis_. In _pallidus_, the geographic range of which is between _guadalupensis_ and _hartwegi_, there are different patterns that are in various degrees intermediate between those described immediately above for _hartwegi_ and _guadalupensis_. _Pattern on Side of Head_ The change in pattern (Fig. 6) and its contrast with the ground color on the side of the head parallels the sequence of changes in pattern on the snout. The pattern on the side of head contrasts with the ground color and consists of dark markings below the eye and on the neck, an indication of a postlabial stripe, and a pale, dark-bordered postocular stripe that may be variously interrupted (_spinifer_ and _hartwegi_; _asper_ usually has uninterrupted postocular and postlabial stripes that unite on the side of the head). The pattern is contrasting but variable in _pallidus_. _T. s. emoryi_ and usually _guadalupensis_ have fewer dark markings, sometimes none, and an interrupted postocular pale stripe that produces a pale blotch just behind the eye. [Illustration: FIG. 6. Pattern on side of head of some American species and subspecies of the genus _Trionyx_. Note the gradual reduction in contrast of pattern and interruption of the postocular stripe from that of _spinifer_ (b) to that of _emoryi_ (f). a. _T. ferox_ (UMMZ 102276, × 1/3) b. _T. spinifer spinifer_ (UMMZ 54401, × 2/3) c. _T. spinifer asper_ (KU 50843, × 2/3) d. _T. spinifer pallidus_ (KU 50830, × 3/4) e. _T. spinifer guadalupensis_ (SM 659, × 2/3) f. _T. spinifer emoryi_ (KU 2922, × 3/4) g. _T. muticus muticus_ (KU 48228, × 2/3) h. _T. muticus calvatus_ (KU 47117, × 2/3) ] [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).