_Geographical variation._--_Trionyx muticus_ shows no obvious character gradients; the variation is mostly discontinuous and unlike that in _T. spinifer_. On the basis of differences in the juvenal pattern and pattern on head, _T. muticus_ can be divided into two subspecies. =Trionyx muticus muticus= Lesueur Midland Smooth Softshell Plates 45, 46, and 53 _Trionyx muticus_ Lesueur, Mém. Mus. Hist. Nat. Paris, 15:263, pl.7, December, 1827. _Trionyx muticus muticus_ Webb, Publ. Mus. Nat. Hist. Univ. Kansas, 11:520, August 14, 1959. _Potamochelys? microcephala_ Gray, Proc. Zool. Soc. London, p. 87, 1864. _Type._--Lectotype, Museum d'Histoire Naturelle, Paris, No. 8813; dried carapace and plastron; obtained from the Wabash River, New Harmony, Posey County, Indiana, by C. A. Lesueur in August, 1827 (Pl. 53). _Range._--Central United States; in the Mississippi River drainage from extreme western Pennsylvania, southern Minnesota and South Dakota south to Tennessee, Louisiana and Oklahoma; streams of the Gulf Coast drainage from the Mississippi River in Louisiana westward into Texas including the Colorado River drainage (see map, Fig. 22). _Diagnosis._--Juvenal pattern of dusky dots and usually short lines or bacilliform marks; ill-defined pale stripes on snout usually evident just in front of eyes; pale postocular stripe lacking thick, black borders that are approximately one-half width of pale stripe (except some in the Colorado River drainage of Texas). _Description._--Plastral length of smallest hatchling, 2.1 centimeters (INHS 3458); of largest male, 14.0 centimeters (CNHM 92003); of largest female, 21.5 centimeters (KU 2308). Juvenal pattern of dusky, grayish marks lacking sharp margins, and usually consisting of both small spots and short streaks or dashes, the former predominating; short streaks or dashes occasionally lacking (TU 14375, Pl. 45, bottom, left; UMMZ 92751); markings variable in number, few and widely spaced, or several and closely approximated (Pl. 45, top, topotypes); pale rim separated from ground color by ill-defined, dusky margin; pattern on adult males well-defined resembling that of hatchlings (TU 16172.1, 16173), scarcely discernable (TU 13294), or absent (TU 1242); mottled and blotched pattern on carapace usually contrasting in large females.