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Much more exclusively aquatic than the Ducks, the Family before us, as their name imports, are remarkable for the readiness and frequency with which they descend beneath the surface of the water, and for the great length of time during which they can remain immersed. They have the beak narrow, straight, and sharp pointed; the head small; the wings short and hollow; the legs, placed very far behind, near the extremity of the body, are flattened sidewise so as to present a thin edge before and behind; the toes armed with broad flat nails. In one genus the toes are united by a membrane, and there is a short tail; in the other two the toes are divided midway to the base, but are margined with broad oval membranes, and there is no vestige of a tail. The latter chiefly affect fresh waters, the former reside upon the ocean.

The backward position of the feet in these birds, while it renders them powerful and fleet swimmers and divers, greatly diminishes their ability for walking. Indeed they scarcely walk at all, for though they can shuffle along awkwardly in an erect position, it is only for a few steps, when they fall down upon their breast, or else remain sitting erect, supported upon their broad feet as a base, the whole tarsus resting on the ground. Their powers of flight are nearly as limited: but under the surface of the water the wings are expanded and used effectually as fins.