The Collaborative International Dictionary of English v.0.48:

Sirenian \Si*re"ni*an\, n. (Zool.)
   Any species of Sirenia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sirenian
    n 1: any of two families of large herbivorous aquatic mammals
         with paddle-shaped tails and flipper-like forelimbs and no
         hind limbs [syn: sea cow, sirenian mammal, sirenian]