This family contains two modern species, dugongs and Steller's sea cow, the latter unfortunately now extinct. Dugongs are found along the coasts of east Africa, the Red Sea, and across most of coastal Asia through the Philippines (but not as far north as Japan), and Australia. Stellar's sea cow lived in the Bering Sea.
Dugongs are large mammals, weighing up to around 400 kg and measuring up to 3.5 m in length. Sea cows were immense, reaching almost 8 m in length and weighing around 5000 kg. Dugongids lack the vestigial nails on their flippers that are possessed by manatees. Also unlike manatees, their tail flukes are deeply notched, not rounded, and their upper lip, while massive, is not as deeply split as is the lip of manatees.
The skulls and teeth of dugongs are unmistakable. The premaxillae are very large, and the entire rostrum is bent sharply downward. Nasals are lacking entirely. The jugal is expanded below the orbit and actually comes into contact with the premaxilla. The supraorbital region of the frontal is not expanded into a shelf over the orbit, and the frontal itself is broader than long, not extended as in manatees. Dugongs have a pair of upper incisors on each side of the jaw. The anterior incisor is vestigial. The second incisor is tusklike and exposed in males, but buried in the premaxilla in females. Lower incisors and canines are vestigial, sometimes present but usually missing and recognized only by their alveoli. The cheekteeth are columnar, with simple, flat crowns. As in the manatee, the cheekteeth move forward in the jaw as the animal ages, eventually dropping out anteriorly. The full dental formula is 2/3, 0/1, 3/3, 3/3 = 36, although of course this number of teeth is never seen in a single specimen.
Sea cows lacked teeth but instead had rough plates between their jaws. Some individuals had small nasals. The rostrum was large and narrow as in dugongs, but only slightly bent downward.
Dugongs are bottom-feeders, grazers on a number of species of aquatic plants. They usually rip the plant from the substrate, swish it gently to remove sand, then eat it. They sometimes pick a number of plants before beginning to eat, stacking them in calm water near shore. They "walk" on their pectoral fins as they feed, leaving distinctive trails on the bottom substrate. They are found only in shallow, coastal habitats, where they occur singly or in small groups of 3-6 members. Their dives are normally short, lasting little more than a minute, in contrast to the long dives of manatees. Not much is known about the behavior of Stellar's sea cows. They also lived in shallow coastal water, feeding on vegetation, sometimes in very large aggregations.
Dugongs are hunted for their meat and leather. Oil rendered from their leather has is said to have curative powers. Sea cows were prized also for meat and leather. Their aggregations and feeding behavior made them easy targets for hunters and sailors, and they were extinct a mere 30 years after their discovery.