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. [Illustration: 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).