The Collaborative International Dictionary of English v.0.48:

Sirenia \Si*re"ni*a\, n. pl. [NL.] (Zool.)
   An order of large aquatic herbivorous mammals, including the
   manatee, dugong, rytina, and several fossil genera.
   [1913 Webster]

   Note: The hind limbs are either rudimentary or wanting, and
         the front ones are changed to paddles. They have horny
         plates on the front part of the jaws, and usually
         flat-crowned molar teeth. The stomach is complex and
         the intestine long, as in other herbivorous mammals.
         See Cetacea
   (b) .
       [1913 Webster]

WordNet (r) 3.0 (2006):

Sirenia
    n 1: an animal order including: manatees; dugongs; Steller's sea
         cow [syn: Sirenia, order Sirenia]