The Mergansers, or Fish Ducks, comprise the remaining subfamily (Mergince) of the great group of Ducks. They are fish-eating Ducks with slender bodies, long Grebe-like necks, and long, narrow, compressed bills, which may be longer or shorter than the head. The lower mandible is without lamelle along the sides, but is provided with a series of distinct tooth-like serrations along the upper edge, as are also the edges of the upper mandible. As much of their food is procured by diving, they are provided with strong feet such as characterize the sea Ducks, just considered. The nine recognized species are placed by some students in a single genus, but by others, and perhaps better authorities, they are separated into three fairly well marked genera. That the group is an old one is shown by the fact that two genera, each with a single species, are known from the Miocene beds of Patagonia.