When the incisors are present, there is a space between them. The cheek teeth, which are arranged in a continuous series, number from 3 to 10 in each half of each jaw. The genera differ in a number of ways. In Dugong and Hydrodamalis the tail fin is deeply notched, but in Trichechus the tail fin is more or less evenly rounded. The upper lip is more deeply cleft in manatees than in Dugong and Hydrodamalis.
Sirenians are solitary, travel in pairs, or associate in groups of three to about six individuals.
Generally slow and inoffensive, they spend all their life in the water. They are vegetarians and feed on various water plants. The only reliable observations of nursing in manatees, however, have revealed that the young suckle while the mother is underwater in a horizontal position, belly downward. The Sirenia often are classified together with the Proboscidea and Hyracoidea in the mammalian superorder Paenungulata. The geological range of the order Sirenia is early Eocene to Recent.