The genus _Megalonyx_, as is well known, owes its name and the discovery of the fossil remains on which it was founded, to the celebrated Jefferson,[31] formerly President of the United States. Cuvier, from an examination of a single tooth, and the casts of certain bones of the extremities, especially the terminal ones, determined the ordinal affinities of this remarkable extinct quadruped.[32] But while he retained, the name of _Megalonyx_, and used it in a generic sense, Cuvier offered no characters whereby other fossil remains might be generically either distinguished from, or identified with the _Megalonyx Jeffersonii_, unless, among such remains there happened to be a tooth, or a claw exactly corresponding with the descriptions and figures in the _Ossemens Fossiles_; and when, of course, a specific identity, and not merely a generic relationship would be established. The greater part of Cuvier’s chapter on _Megalonyx_ is devoted to the beautiful and justly celebrated reasoning on the ungueal phalanx, whereby it is proved to belong, not to a gigantic Carnivore of the Lion-kind, as Jefferson supposed, but to the less formidable order of Edentate quadrupeds; and Cuvier, in reference to the tooth,—the part on which alone a generic character could have been founded,—merely observes that it resembles at least as much the teeth of one of the great Armadillos, as it does those of the Sloths.[33] In the last edition of the _Rêgne Animal_, Cuvier introduces the _Megatherium_ and _Megalonyx_, between the Sloths and Armadillos; but alludes to no other difference between the two genera than that of size,—“l’autre, le _Megalonyx_, est un peu moindre.” (p. 226.) Some systematic naturalists, as Desmarest, and Fischer, have, therefore, suppressed the genus, and made the _Megalonyx_ a species of _Megatherium_ under the name of _Megatherium Jeffersonii_. The dental characters of the genus _Megatherium_ are laid down by Fischer[34] as follows:—“_Dent. prim, et lan. ⁰⁄₀., molar es ⁴⁄₄⁴⁄₄, obducti, tritores, coronide nunc planâ transversim sulcatâ nunc medio excavatâ marginibus prominulis._” That _Megalonyx_ had the same number of molares as _Megatherium_, (supposing that number in the Megathere to be correctly stated, which it is not,) is here assumed from analogy, for neither Jefferson, Wistar, nor Cuvier,—the authorities for _Megalonyx_ quoted by Fischer—possessed other means of knowing the dentition of that animal than were afforded by the fragment of a single tooth.