The third and last subfamily is that of the otters, or _Lutrinae_, in which the feet (with the exception of the hind pair in the sea-otter) are short and rounded, with the toes webbed, and the claws small, curved and blunt. The head is broad and much depressed. The upper posterior cheek-teeth are large and quadrate. The kidneys are conglomerate. Habits aquatic.
In the true otter of the genus _Lutra_ the dentition is i. 3/3, c. 1/1, p. 4/3, m. 1/2; total 36. Upper sectorial with a trenchant tricusped blade, and a very large inner lobe, hollowed on the free surface, with a raised sharp edge, extending along two-thirds or more of the length of the blade. Upper molar large, with a quadricuspidate crown, broader than long. Skull broad and depressed, contracted immediately behind the orbits; with the facial portion very short and the brain-case large. Vertebrae: C. 7, D. 14-15, L. 6-5, S. 3, Ca. 20-26. Body very long. Ears short and rounded. Limbs short. Feet completely webbed, with well-developed claws on all the toes. Tail long, thick at the base and tapering, rather depressed. Fur short and close.
Otters are more or less aquatic, living on the margins of rivers, lakes, and in some cases the sea; are expert divers and swimmers, and feed chiefly on fish. They have an extensive geographical range, and so much resemble each other in outward appearance, especially in the nearly uniform brown colouring, that in some cases the species are by no means well-defined. The Brazilian otter (_L. brasiliensis_) is a very large species from Brazil, Demerara and Surinam, with a prominent ridge along each lateral margin of the tail. In two small species the feet are only slightly webbed; claws exceedingly small or altogether wanting on some of the toes; the first upper premolar very small, sometimes wanting; and the molars very broad and massive. The species in question are _L. inunguis_ of South Africa, and _L. leptonyx_ or _cinerea_ of India, Java and Sumatra, and have been separated as a distinct genus, Aonyx.
The sea-otter, _Latax_ (or _Enhydra_) _lutra_, with a dentition of i. 3/2, c. 1/1, p. 3/3, m. 1/2, total 32, differs from other Carnivora in having but two incisors on each side of the lower jaw, the one corresponding to the first (very small in the true otters) being absent. Though the molar teeth generally resemble those of _Lutra_ in their proportions, they differ in the exceeding roundness and massiveness of their crowns and bluntness of their cusps. Feet webbed; fore-feet short, with five subequal toes, with short compressed claws; hind-feet very large, depressed and fin-like, their phalanges flattened as in seals. The fifth toe the longest and stoutest, the rest gradually diminishing in size to the first, all with moderate claws. Tail moderate, cylindrical (see OTTER).