Now the almost entire lower jaw about to be described offers, in so far as respects the general form and structure of the teeth, the same kind and degree of correspondence with the _Megatherium_, as does the _Megalonyx Jeffersonii_ of Cuvier: and, what is only probable in that species, is here certain, viz., an agreement with the Megatherium in the class, viz. _molares_, to which the teeth exclusively belong. The question, therefore, on which I find myself, in the outset, called upon to come to a decision is, as to the preference of the mode of viewing the subject of the generic relationship of the _Megalonyx_ adopted by Desmarest, Fischer, &c., or of that, on which Cuvier, and after him Dr. Harlan, have practically acted: whether, in short, the genus _Megatherium_ is to rest upon the more comprehensive characters of kind and general structure of the teeth, or upon the more restricted ones, of form and such modifications in the disposition and proportions of the component textures of the tooth, as give rise to the characteristic appearances of the triturating surface of the crown. With respect to existing Mammalia, most naturalists of the present day seem to be unanimous as to the convenience at least of founding a generic or subgeneric distinction on well-marked modifications in the form and structure of the teeth, although they may correspond in number and kind, in proof of which it needs only to peruse the pages of a _Systema Mammalium_ which relate to the distribution of the Rodent Order. According to this mode of viewing the logical abstractions under which species are grouped together, the extinct Edentate Mammal discovered by Jefferson must be referred to a genus distinct from _Megatherium_, and for which the term _Megalonyx_ should be retained. This will be sufficiently evident by comparing the descriptions given by Cuvier of one of the teeth of the _Megalonyx Jeffersonii_, and by Dr. Harlan of a tooth of his _Megalonyx laqueatus_, with those of the _Megatherium_ which have been published by Mr. Clift. The fragment of the molar tooth of the _Megalonyx Jeffersonii_, described and figured in the _Ossemens Fossiles_, seems to have been implanted in the jaw, like the teeth of the _Megatherium_, by a simple hollow base similar in form and size to the protruded crown: its structure Cuvier describes as consisting of a central cylinder of bone enveloped in a sheath of enamel.[35] The transverse section of this tooth presents an irregular elliptical form, the external contour being gently and uniformly convex, the internal one, undulating; convex in the middle, and slightly concave on each side, arising from the tooth being traversed longitudinally on its inner side by two wide and shallow depressions.