_Remarks._--_Trionyx muticus_ generally has been considered a distinct species since its description by Lesueur (1827:263-66, Pl. 7); Wied-Neuwied (1865:53), at least, questioned the identity of _muticus_, believing it to be based on a secondary sexual difference of _T. spiniferus_. Lesueur did not designate a type in the description, and mentioned that he had seen only three specimens (_op. cit._:264). Stejneger (1944:17-18) discussed two mounted specimens (Nos. 787 and 788) in the Natural History Museum at Paris, and mentioned that No. 787 was designated "... as the type on the printed label (although presumably not done by Lesueur)." Dr. Jean Guibé (_in litt._ September 24, 1959) informed me that Nos. 787 and 788 are numbers without value and correspond, respectively, to catalog numbers 8813 and 8814. In addition, the Museum possesses an alcoholic specimen, No. 564, obtained by Lesueur from the Wabash River, that seems to have been acquired by the museum after the publication of the original description. No. 8813 is regarded as a lectotype. Gray (1864:87) described the species _microcephalus_ and questionably included it in the genus _Potamochelys_ Fitzinger, 1843; the locality was stated as "Sarawak (Wallace)." Gray especially noted the small elongate head and believed that the acquisition of adult specimens would prove that it belonged to a new genus. Later, Gray (1869:221) proposed the generic name _Callinia_ as a new name for _Aspidonectes_ as understood by Agassiz (1857:403). Gray referred _microcephala_ to _Callinia_ (_op. cit._:214, 222) and recognized also _Amyda mutica_ (_op. cit._:212). Baur (1888:1121) remarked that "_Callinia microcephala_ Gray, of the British Museum, with the locality Sarawak, is _Amyda mutica_ Les." The species _microcephalus_ has since been considered a synonym of _Trionyx muticus_. Schmidt (1953:110) designated the type locality as New Harmony, Indiana. Müller (1878:641) listed the species _Trionyx muticus_ from México as follows: "*b. in Alcohol. Mexico. 1872. [2]." Smith and Taylor (1950:18, footnote) wrote that the record required confirmation. Webb and Legler (1960:24) questionably referred this record to the synonomy of _T. ater_, which resembles _muticus_. _T. muticus_ is not known to occur in México. According to Dr. Lothar Forcart (_in litt._) of the Naturhistorische Museum in Basel, Switzerland, only one specimen on which Müller based his record is extant. My examination of this specimen reveals that it is a hatchling _T. s. emoryi_, plastral length 3.5 centimeters, bearing catalog number 1032; there are no additional data of collection.