_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 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).