226 THE ANATOMY OF VERTEBRATED ANIMALS. 



downward, nor does it send a process upward in front of the 

 tibia. 



6. The astragalus remains quite free from the tibia. 



In all these respects, the leg of any existing bird (see 

 Fig. 78) is very strikingly contrasted with that of the 

 reptile : 



1. The proximal end of the tibia is produced forward and 

 outward into an enormous cnemial crest, in all walking and 

 swimming birds (Fig. 78, A.) ; and, on the outer side, there 

 is a strons: rid^-e for the fibula. 



2. When the posterior edges of the condyles of the tibia 

 rest upon a flat surface, the one flat face of the distal end of 

 the bone looks outward as well as forward, and the other in- 

 ward as well as backward. Further, the long axis of the dis- 

 tal end is inclined, at an angle of 45° to the flat surface, from 

 within and in front, backward and outward, thus exactly re- 

 versing the direction in the reptile. 



3. There is a deep longitudinal depression on the anterior 

 face of the distal end of the tibia, which receives an ascendinjr 

 process of the astragalus. 



4. The distal end of the fibula is a mere style, and does not 

 articulate with the astragalus. 



5. The astragalus is a much-depressed bone, with a concave 

 proximal, and a convex, pulley-like, distal, surface. A process 

 ascends from its front margin in the groove on the front face 

 of the tibia. This process is comparatively short, and perfo- 

 rated by two canals for the tibialis anticus and extensor com- 

 tiiunis, in the Fowl ; while in the Ostrich and Emeu it is ex- 

 tremely long and not so perforated. 



6. The astragalus becomes anchylosed with the tibia 

 (though it remains distinct for a long time in the Ostrich and 

 lihea^ and in some breeds of fowls). 



In the Ornithoscelida : 



1. Tlaere is a great cnemial crest and a ridge for the fibula. 



2. The disposition of the distal end of the tibia is literally 

 that observed in the Bird. 



3. There is a fossa for the reception of the ascending pro- 

 cess of the astragalus. 



4. The distal end of the fibula is much smaller than the 

 proximal, though not so slender as in Aves. 



5. The astragalus is altogether similar to that of a bird, 

 with a short ascending process. 



6. The astragalus appears to have remained distinct from 

 the tibia throughout life in Igiianodon, Megalosaurus^ and 



