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.
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).
- T. ferox (UMMZ 102276, × 1/3)
- T. spinifer spinifer (UMMZ 54401, × 2/3)
- T. spinifer asper (KU 50843, × 2/3)
- T. spinifer pallidus (KU 50830, × 3/4)
- T. spinifer guadalupensis (SM 659, × 2/3)
- T. spinifer emoryi (KU 2922, × 3/4)
- T. muticus muticus (KU 48228, × 2/3)
- T. muticus calvatus (KU 47117, × 2/3)