II SKELETONS OF BIRD AND REPTILE 17 
not,as in mammals and most reptiles, entirely of 
bone, and it has vanished. But ankylosis or fusion is 
perhaps the most marked characteristic of a bird’s 
leg. There are the same bones as in the lizard’s leg, 
if we could only see them—viz.: Femur (FE, fig. 2) 
or thigh-bone, Tibia (T), Fibula (F), two rows of 
Tarsals or ankle-bones (TA), four of the lizard’s five 
Metatarsals (MT), though of one of the four only the 
farther end (MT,, fig. 2) remains, and four of his five 
Fic. 6.—Hatteria Lizard’s left hind foot. 
DI, 2, 3) 4, 5, digits; F, fibula; mT, metatarsals; tT, tibia ; TA, two bones fixed, 
represents near row of tarsals ; TAg, distant row. 
digits. The Femur has not undergone so much 
change, but the Tibia and Fibula (fig. 3) are very 
different from the corresponding bones in reptiles. 
The latter has nearly vanished ; it is a slender, almost 
needle-like bone, attached to the side of the Tibia and 
not reaching to its farther end. In many mammals 
too the Fibula is but a remnant. The way to make 
certain, in the skeleton of any animal whatever, which 
bone is the Tibia and which the Fibula, is to imagine 
the limb extended, as it is in the lizard, outwards from 
the body ; then the Tibia is praeaxial and the Fibula 
Cc 
