The La Plata Otter in its general colouring is of a somewhat deeper hue than the European species, the cheeks and throat instead of being nearly white are of a pale brown colour; the tail is longer in proportion, and tapers more gradually; the tip of the muzzle is naked, but the hairless portion is less than in that species, the boundary line between the naked part and the hair of the top of the muzzle forming almost a semicircle; the retiring extremities of this line touch the posterior angle of the nostril on each side, whereas in the common otter the boundary line of the hair of the muzzle is of a ｗ-like form. The skull is figured in Plate 35, figs. 4, _a_, _b_, _c_, and _d_, and is compared with that of _L. Chilensis_ in the next description. “This specimen was killed by some fishermen a few miles from Maldonado, near the mouth of the estuary of the Plata, where the water is quite salt. I am not, however, by any means sure that it may not be a fresh-water species, which had wandered from its proper station; in the same manner as not unfrequently is the case with the _Hydrochærus Capybara_. I am indebted to Mr. Chaffers, the master of the Beagle, for having kindly presented me with this specimen.”—D. 2. LUTRA CHILENSIS. Lutra Chilensis, _Bennett_, Proceedings of the Committee of Science and Correspondence of the Zoological Society of London for 1832, p. 1. _L. fusca; vellere mediocri, laxo et sub-extante; mento, gulâ, et faciei lateribus, pallidè fuscis; pedibus saturatè fuscis; corporis pilis ad apicem pallidè fuscis; caudá mediocri; rostri apice calvo._ DESCRIPTION.—This species scarcely equals a full grown European otter in size. It is of a brown colour throughout; the cheeks, chin, and throat, being slightly paler, and the feet of a deeper tint, than the other parts. The fur is moderately long, rather harsh to the touch, and semi-erect: the under fur is abundant, and of a soft and silk nature. The hairs of the ordinary fur are deep brown, but tipped with a very pale brown colour. The hairs of the tail, like those of the body, are harsh and semi-erect; towards the apex, those on the upper and under part are in a slight degree shorter than those at the sides, and lie closer to the skin; these differences, however, are not very apparent on the upper side, though distinct on the under. The feet are naked beneath, with the exception of the posterior half of the tarsus. The hair of the muzzle extends only down to the posterior angle of the nostrils, where it terminates in a straight line, leaving the tip of the muzzle naked. In. Lines. Length from nose to root of tail 31 0 of tail 14 3 Habitat, Chonos Archipelago, (_January._.)