pygopodes

from The Century Dictionary.

• An order of swimming and diving birds, the rump-footed birds, having the legs inserted very far back, and buried in the common integument of the body nearly to the heel.

from the GNU version of the Collaborative International Dictionary of English.

• noun plural (Zoöl.) A division of swimming birds which includes the grebes, divers, auks, etc., in which the legs are placed far back.