This fossil is referred by Dr. Harlan to his _Megalonyx laqueatus_; but, pending the absence of other proof of the identity of species, in which, as may be seen by comparing fig. 2, with fig. 4, in Pl. XVII., the teeth differ widely in form, it would be obviously hazardous to adopt such an approximation on hypothetical grounds.[40] In order, however, to obtain more satisfactory evidence of the nature and amount of the difference between the _Megalonyx laqueatus_, and the allied animal represented by the above-described fragment of lower jaw, I wrote to my much respected friend M. LAURILLARD, requesting him to send me a sketch of the teeth in the cast of that lower jaw, which had been transmitted from New York to the Garden of Plants. With full confidence in the characteristic precision and accuracy of the drawing with which I have been obligingly favoured by M. Laurillard, I am disposed to regard the amount of difference recognizable in every tooth in the lower jaw in question (fig, 3 and 4,) as compared with the molar tooth either of _Megalonyx Jeffersonii_ (fig. 1,) or _Meg^x. laqueatus_ (fig. 2) to be such as to justify its generic separation from _Megalonyx_ on the same grounds as _Megalonyx_ is distinguished from _Megatherium_, and for the subgenus of Megatherioid Edentata, thus indicated, I would propose the name of MYLODON.[41] The species of which the fossil remains are described by Dr. Harlan may be dedicated to that indefatigable Naturalist who has contributed to natural science so much valuable information respecting the Zoology, both recent and fossil, of the North American continent. The fossil about to be described represents a second and smaller species of the same genus, and I propose to call it _Mylodon Darwinii_, in honour of its discoverer, of whose researches in the Southern division of the New World it forms one of many new and interesting fruits. This fossil was discovered in a bed of partly consolidated gravel at the base of the cliff called Punta Alta, at Bahia Blanca in Northern Patagonia: it consists of the lower jaw with the series of teeth entire on both sides: but the extremity of the symphysis, the coronoid and condyloid processes, and the angular process of the left ramus, are wanting. The teeth are composed, as in _Bradypus_, _Megatherium_ and _Megalonyx_, of a central pillar of coarse ivory, immediately invested with a thin layer of fine and dense ivory, and the whole surrounded by a thick coating of cement.