The Collaborative International Dictionary of English v.0.48:

Mergus \Mergus\ prop. n.
   A genus of ducks consisting of mergansers.

   Syn: genus Mergus.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Mergus
    n 1: mergansers [syn: Mergus, genus Mergus]