Gray (1844:48) proposed the name _Tyrse argus_ for a specimen reported to have come from Sierra Leone, West Africa; later (1855:68), he referred the species to the genus _Trionyx_. After comparison with a specimen of _T. spiniferus_ Lesueur, Gray (1864:89) was "doubtful whether there must not have been some confusion about the habitat of the specimen [which formed the basis of the description of _Tyrse argus_], and whether it is not more probably a North American species." The same author (1869:222; 1870:109) listed _Tyrse argus_ as a synonym of _Callinia spinifera_ (= _Trionyx spiniferus_ Lesueur). Agassiz (_op. cit._:406-07) described _Aspidonectes nuchalis_ on the basis of three adults from the Cumberland River and a number of young from the headwaters of the Tennessee River. Boulenger (1889:245, footnote 2) suggested that the status of _A. nuchalis_ required further investigation. The species was not generally recognized after the turn of the century. Barbour and Loveridge (1929:226) listed MCZ 1908 (one of the juveniles) and 1623-25 as cotypes. Stejneger (1944:52) showed that _nuchalis_ was not distinguishable from _T. s. spinifer_, and (_op. cit._:49) listed MCZ 1623-25 as cotypes. Schmidt (1953:110) restricted the type locality to the Cumberland River, near Nashville, Tennessee. Agassiz (_loc. cit._) mentioned that _nuchalis_ "differs strikingly from Asp. spinifer in the much more elongated form of the male, and in the great development of the marginal spines and of the tubercles upon the carapace, ... But the most prominent specific character consists in the marked depressions on either side of the blunt median keel, and also in the triangular dilation of that keel behind the front margin of the carapace." These characters seem to be of no taxonomic worth. I have seen three syntypes (MCZ 1623-25) that undoubtedly correspond to the three adult specimens mentioned by Agassiz. All are females, measuring 19.5, 22.0, and 19.0 centimeters, respectively, in plastral length, and lack a contrasting mottled pattern on the carapace; the juvenal pattern is obscured, except for blackish spots at the edge of the carapace on MCZ 1625, and parts of an ocellus on MCZ 1624. The dorsal surfaces of the limbs are boldly marked. MCZ 1623, showing the diagnostic feature mentioned by Agassiz, is photographed by Stejneger (_op. cit._:Pls. 14, 15), and may be regarded as the lectotype of _Aspidonectes nuchalis_ Agassiz. MCZ 1908 is one of the young syntypes mentioned by Agassiz, and is referable to _spinifer_. The juvenal pattern consists of spots and ocelli; the plastron measures 3.1 centimeters in length, and the carapace 4.2 centimeters.