In the squirrels and porcupines the tibia and fibula are distinct, but in rats and hares they are united, often high up. The hind foot is more variable than the front one, the digits varying in number from five, as in squirrels and rats, to four, as in hares, or even three, as in the capybara, viscacha and agouti.
The tibia and fibula are united inferiorly, the tympanic bulla is hollow and the infra-orbital foramen narrow.
In both groups the tibia and fibula are separate.
C. Marsh, by finding the imperfect fossilized tibia of a bird in the middle cretaceous shale of Kansas, Marsh, began a series of wonderful discoveries of great im portance to ornithology.
Order Colymbiformes.-Plantigrade, nidifugous, aquatic. All toes webbed, fourth largest, hallux short; metatarsus laterally compressed; tibia with high, pyramidal crest.
Radius confluent with ulna, and tibia with fibula; tarsus (astragalus and calcaneum) elongate, forming an additional segment in the hind limb.
The orbit is surrounded by a bony ring; the ulna and radius in the fore, and the tibia and fibula in the hind-limb are united, and the feet are of the types described above.
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.
In the Bathyergoidea the scaphoid and lunar of the carpus are separate, the tibia and fibula united and the clavicles normal.
The femur has a small third trochanter, the radius and ulna and tibia and fibula are respectively separate, at least in the young, and the fibula articulates with the astragalus.