Snout
The snout ([Fig. 12]) is tubate having terminal nostrils separated by a vertical septum. One of the principal characters distinguishing T. ferox and T. spinifer from T. muticus is a lateral, whitish ridge projecting from each side of the nasal septum (hereafter referred to as septal ridges but often referred to in the literature as a papilla). The shape of the end of the snout is truncate in T. ferox and T. spinifer, and the nostrils are larger than in T. muticus. In muticus the snout usually terminates somewhat obliquely, and the nostrils tend to be slightly inferior; also, the end of the snout is usually rounded and somewhat pointed, causing the nostrils to be visible in lateral view. Some T. muticus do not differ markedly from ferox or spinifer in shape of the end of the snout. Stejneger (1944:14) mentioned indication of a septal ridge that did not reach the opening of the nostril in muticus. I have slit the outer edge of the nostril on several specimens of muticus, and have not noticed an indication of a septal ridge.
Fig. 12. Shape of snout in T. spinifer (left, a-d, from KU 46907) and T. muticus (right, e-h, from KU 48236). Lateral views—a, e (× 1); anterior views—b, f (× 5); dorsal views—c, g (× 2.5); ventral views—d, h (× 2.5).
Tuberculation
Tubercles or obtuse prominences occur on the anterior edge of the carapace ([Fig. 8]) or on the dorsal surface of the carapace. Trionyx muticus lacks tubercles, although some individuals show shallow, widely spaced wrinkles that suggest prominences on the anterior edge of the carapace. Both sexes of T. ferox have prominences, resembling flattened hemispheres, on the anterior edge of the carapace and in the nuchal region. Large females of ferox have obtuse prominences in the center of the carapace posteriorly, some of which are often arranged in longitudinal rows. The surface of the carapace in both sexes of T. ferox has small closely-set, blunt tubercles arranged in rows that resemble longitudinal ridges (most evident in juveniles).
Large females of T. spinifer have obtuse prominences in the center of the carapace posteriorly, some of which in many specimens are arranged in longitudinal rows; I cannot discern any correlation of number or arrangement of prominences with size in spinifer or ferox. The carapace in adult males of spinifer bears small, sharp tubercles that make the surface feel like sandpaper. The tubercles on the anterior edge of the carapace in adults of both sexes vary from round to equilateral and conical to low and flattened (see comments on tuberculation under subsection entitled "Geographic Variation"). Some large females of the same subspecies have tubercles on the anterior edge of the carapace that may be conical (higher than wide) or equilateral. The difference in shape of the tubercles seems not to be correlated with size because one T. s. pallidus, 30.5 centimeters (TU 13212) has prominent but blunted and equilateral tubercles, whereas, another female of pallidus, 20.8 centimeters (TU 13210), from the same locality has higher, conical tubercles. The blunted, equilateral tubercles may be the result of environmental wear, or the difference in shape of tubercles may be due to individual variation.