Order Colymbiformes.-Plantigrade, nidifugous, aquatic. All toes webbed, fourth largest, hallux short; metatarsus laterally compressed; tibia with high, pyramidal crest.
In both groups, for instance, the lower part of the hind-leg is formed by a long, slender cannon-bone, or metatarsus, terminating inferiorly in triple condyles for the three long and sharply clawed toes, the resemblance being increased by the fact that in both cases the small bone of the leg (fibula) is fused with the large one (tibia).
It may also be noticed that in mammals and birds which hop on two legs, such as jerboas, kangaroos, thrushes and finches, the proportionate length of the thigh-bone or femur to the tibia and foot (metatarsus and toes) is constant, being 2 to 5; in animals, on the other hand, such as hares, horses and frogs, which use all four feet, the corresponding lengths are 4 to 7.
Collimate around the sides of the limb and include the whole calcaneus, the distal tibia and the proximal metatarsus.