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.
T. muticus is the smallest species in North America; the maximum size of the plastron in adult males is approximately 14.0 centimeters (16.0 cm. in spinifer) and of adult females 21.5 centimeters (31.0 cm. in spinifer). Males and females of muticus are sexually mature at approximately the same size as some T. s. emoryi; also, the great development of the plastral callosities in muticus corresponds to that in some emoryi. The head is narrower in muticus than in ferox or spinifer. The carapaces of specimens of muticus exceeding plastral lengths of 8.0 centimeters are wider than those of ferox, ater, T. s. emoryi and T. s. guadalupensis of corresponding size. T. muticus differs from ater and three subspecies of spinifer (pallidus, guadalupensis, emoryi) in having the carapace widest at a plane approximately one-half the length of the carapace. The snout is longer in muticus than in ferox and spinifer. T. muticus differs from ferox but resembles spinifer in having a relatively short plastron.
The skulls of muticus differ from those of ferox but resemble those of spinifer in usually having the skull widest at the level of the squamosals. Skulls of muticus resemble those of ferox but differ from those of spinifer in usually lacking a well-developed opisthotic-exoccipital spur. Skulls of muticus are different from those of ferox and spinifer in having the 1) ventral surface of the supraoccipital spine widest proximally, lacking a medial ridge, 2) foramen magnum ovoid, 3) distal part of opisthotic wing truncate, 4) lateral condyle of articular surface of quadrate tapered posteriorly, smaller than medial articular surface, and 5) maxillaries not in contact above premaxillaries.
Plastrons of muticus differ from those of spinifer and ferox in having an [534] obtusely-angled epiplastron, relatively large callosities in adults, and a wide hyo-hypoplastral bridge (in relation to length).
Remarks.—Agassiz (1857:399) regarded Lesueur's Trionyx muticus as the type species of the genus Amyda and the only species known to belong to the genus Amyda. Stejneger (1944:7, 9, 12) proposed the generic name Euamyda as a new name for the North American Amyda mutica as understood by Agassiz. Euamyda was proposed for use only if Agassiz's understanding was found to be correct. Actually, Stejneger thought that the Old World and New World kinds concerned were congeneric, and that the type species of the genus Amyda was the Old World species Amyda javanica Schweigger (= Testudo cartilaginea Boddaert).
If Trionyx muticus Lesueur is considered to be generically distinct from other soft-shelled turtles, Euamyda Stejneger, 1944, is available as a generic name with Trionyx muticus Lesueur, 1827, as the type species (by monotypy).
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
Trionyx muticus Lesueur, Mém. Mus. Hist. Nat. Paris, 15:263, pl. 7, December, 1827.