Size small; head narrow; snout long; ventral surface of supraoccipital spine broad proximally, lacking median ridge; foramen magnum evenly rounded, ovoid; opisthotic-exoccipital spur absent; distal part of opisthotic wing truncate; lateral condyle of articular surface of quadrate tapered posteriorly, smaller than medial articular surface; angle of epiplastron obtuse, approximately 100 degrees; callosity on epiplastron sometimes covering entire surface; bony bridge wide in relation to length. _Description._--Septal ridges absent; external characteristics variable (see accounts of subspecies); range in length, in centimeters, of plastron of ten largest specimens of each sex, (mean follows extremes), males, 11.8-14.0, 12.3; females, 17.7-21.5, 18.9; ontogenetic variation in PL/HW, mean PL/HW of specimens having plastral lengths 7.0 centimeters or less, 4.16, ranging from 7.1 to 13.0 centimeters, 5.82, and, exceeding 13.0 centimeters, 7.04; little ontogenetic variation in CL/CW, mean CL/CW of specimens having plastral lengths 8.0 centimeters or less, 1.15, and exceeding 8.0 centimeters, 1.16; mean CL/PCW, 1.97; mean HW/SL, 1.22; mean CL/PL, 1.39. Greatest width of skull usually at level of squamosal (79%); foramen magnum ovoid; opisthotic-exoccipital spur usually absent (97%); distal part of opisthotic wing truncate, sometimes visible in dorsal view; lateral condyle of articular surface of quadrate tapered posteriorly, smaller than medial articular surface; maxillaries not in contact above premaxillaries; combination of seven neurals, seven pairs of pleurals, and contact of seventh pair of pleurals (38%), or eight neurals, seven pairs of pleurals, and separation of seventh pair of pleurals (41%); angle of epiplastron obtuse, greater than 90 degrees; callosities well-developed, frequently on preplastra and epiplastron of adults. _Comparisons._--The absence of septal ridges distinguishes _muticus_ from _ferox_, all subspecies of _spinifer_, and _ater_ (ridges are reduced in males of _ater_). The smooth anterior edge of the carapace distinguishes _muticus_ from all other American kinds except _ater_ and some individuals of _T. s. emoryi_. _T. muticus_ resembles only _ater_ and _ferox_ in usually lacking a well-defined, contrasting pattern of blackish marks on the dorsal surface of the limbs. _T. muticus_ resembles _ferox_ and differs from _spinifer_ and _ater_ in lacking a gritty or "sandpapery" carapace on adult males. Adult males of _T. muticus calvatus_ and some individuals of _T. m. muticus_ from the Colorado River in Texas further resemble _ferox_ in having postocular stripes with thick black borders.