The Chile Otter was originally described by Mr. Bennett from a specimen presented to the Zoological Society by Mr. Cuming, but as this specimen is a young animal, scarcely half-grown, it does not present some of the characters of the species in so marked a manner as the adult. I have, therefore, availed myself of an adult specimen in Mr. Darwin’s collection, to draw up the above description. Compared with the Common Otter (_Lutra vulgaris_, Auct.) the most striking difference consists in the character of the fur: the hairs instead of being adpressed as in that species, are here semi-erect, and appear as if they had been clipped at the extremity. The fur is of a deeper colour, but has a slightly grizzled appearance, owing to the tip of each hair being of a much paler colour than the remaining part. In the young animal described by Mr. Bennett, (which in weight was probably not more than one-third of that of the present animal) the hairs of the body are of an uniform deep brown colour; hence, if I am right in considering Mr. Darwin’s animal as the same species, it would appear that the grizzled character of the fur is dependent on age. The semi-erect fur will also serve to distinguish the present species from the _Lutra Platensis_; the fur is likewise longer, the tail is shorter, and the feet are smaller in proportion. The most important distinctions, however, are furnished by the skulls; I will, therefore, compare them. The skull of _L. Chilensis_ compared with that of _L. Platensis_, (Plate 35, figs. 4.) when viewed from above, presents but little difference in general form; it is, however, smaller in all its proportions, and the zygomatic arch is a little less convex: the palate is proportionately shorter; the tympanic bullæ are much smaller, less elevated, and wider apart, in which respect there is a greater approximation to the skull of _L. vulgaris_ than to that of _L. Platensis_; but here, the tympanic bullæ are larger than in _L. Chilensis_. Both in _L. Chilensis_ and _Platensis_, the sub-orbital foramina are kidney-shaped, the emarginated portion being downwards, whilst in _L. vulgaris_ they approach somewhat to a triangular figure, the apex being external. In _L. Chilensis_, however, this foramen is comparatively larger than in _L. Platensis_, and the outer portion of the foramen forms the segment of a larger circle than the inner one, whilst in _L. Platensis_ both portions are equal.