_Comparisons._--_Trionyx m. calvatus_ is most closely related to _Trionyx m. muticus_. Both subspecies have the following characteristics: (1) no enlarged tubercles on the anterior edge of the carapace, (2) no ridge projecting from the nasal septum, and (3) a smooth carapace in adult males. These characters distinguish these two subspecies from the several subspecies of _T. spinifer_, and, except for the smooth carapace in adult males, from _T. ferox_. Another feature of _T. m. calvatus_ and _T. m. muticus_, not known to be definitive or diagnostic but noticed on occasion, is the pale orange cast, in life, of the dorsal surface of the carapace and soft parts of the body in young of these turtles. The spotted pattern of juveniles of _calvatus_ is easily distinguished from the pattern of _muticus_ (small dots, streaks and dashes) figured by Agassiz (1857, vol. 2, pt. 3, pl. 6, fig. 6), Smith (1950:154, fig. 104), Conant (1938:192, pl. 21, fig. 1; 1958, pl. 11, opposite p. 94), and Cahn (1937:177, pl. 24C). Unfortunately, the distinctive dorsal spotting in young _calvatus_ becomes obscure or absent in some adults of both sexes. Spotting in large males is not so well-defined as in juveniles; it may be absent (TU 17306.3), or indicated by two obscure spots (KU 17117), but is usually evident, at least posteriorly. The spotted pattern is absent in large females, which have a pale, mottled and blotched pattern of lichen-like figures; dorsal spots are obscure in TU 17305 (length of plastron, 56 mm). Two additional features are, so far as known, universal in _calvatus_; these are: (1) the absence of striping on the dorsal surface of the snout, and (2) the presence of thick, black borders of the postocular stripe in adult males. These features have also been observed in some specimens of _muticus_; their presence in _muticus_ cannot be properly evaluated at this time, and is seemingly not due to individual variation. These two characters, however, coupled with the distinctive juvenile pattern of spots, serve, in combination, to distinguish _calvatus_ from _muticus_.