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_.

_Discussion._--The two populations are recognized as subspecies
because: (1) there is close resemblance, (2) the diagnostic characters
pertaining to pattern are few and superficial, and (3) the geographic
ranges are allopatric, but juxtaposed