The principal difference in the dentition of the La Plata and the Chile otters, consists in the comparatively smaller size of the posterior molars, both of the upper and lower jaws, of the latter species. In the upper jaw, the “carnassière” has its inner lobe, approaching somewhat to a triangular form, whereas in _L. Platensis_ it is broader and almost semicircular. In the lower jaw, the last molar but one has the inner lobe much smaller than the middle outer lobe, whilst in _L. Platensis_ these two lobes are of nearly equal size and elevation. Other points of dissimilarity will be perceived in the annexed table of admeasurements. _L. _L. Chilensis._ Platensis._ In. Lines. In. Lines. Whole length of skull 3 9¾ 4 2½ Greatest width 2 6⅓ 2 10⅓ Width of skull from the apex of one mastoid process to the opposite 2 3¼ 2 8¼ Length of palate 1 6 1 10 Breadth of palate between the posterior molars 7¾ 7¾ Length from last molar to posterior margin of palate 3⅓ 5½ from base of canine to hinder part of last molar 11⅔ 1 1⅓ of carnassière 5 5⅔ Width of do. 5 6¼ Length of last molar 2¾ 3½ Width of do. 4⅓ 5⅔ Length of ramus of lower jaw 2 4½ 2 8¾ from canine to hinder portion of last molar (lower jaw) 1 2¼ 1 4¼ of last molar but one (lower jaw) 5¾ 6⅔ Width of do. 2⅔ 3½