There is only 1 living species in the family Dugongidae. The other recent member, Steller's sea cow (Hydrodamalis gigas) has been extinct since 1768. The dugong is tropical and subtropical, but Steller's sea cow was an inhabitant of cold temperate to subarctic waters. In members of this family, the flattened tail is expanded into flukes, similar to those of cetaceans. Other caracteristics include a rostrum that is deflected downwards, the presence of erupted tusks in males (dugongs only; Steller's sea cow had no teeth), a more streamlined body than those of manatees, and the absence of nails on flippers. <123>
The genera Eosiren, Eotheroides, and Prototherium have been assigned to Halitheriinae in the past, but recent cladistic analysis recovers these genera as basal to the clade formed by Trichechidae and Dugongidae. Moreover, Halitheriinae is paraphyletic with respect to Dugonginae and Hydrodamalinae, and further use of the name should be discontinued because the type genus is based on a non-diagnostic tooth.
^Jorge Vélez-Juarbe and Daryl P. Domning (2015). "Fossil Sirenia of the West Atlantic and Caribbean region. XI. Callistosiren boriquensis, gen. et sp. nov.". Journal of Vertebrate Paleontology. in press. doi:10.1080/02724634.2014.885034.