264 THE ANATOMY OF VERTEBEATED ANIMALS. 



of the bone looks outwards as well as foi-wards, and the 

 other inwards as weU as backwards. Further, the long axis 

 of the distal end is inclined, at an angle of 45° to the flat 

 surface, from within and in front, backwards and outwards, 

 thus exactly reyersing 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 ascend- 

 ing 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 con- 

 cave 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 perforated by two canals for the tibialis anticus and 

 extensor communis, in the Fowl ; while in the Ostrich and 

 Emeu it is extremely long and not so perforated. 



6. The astragalus becomes ankylosed with the tibia 

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

 and Rhea, and in some breeds of fowls). 



In the Ornithoscelida : — - 



1 . There is a gi-eat cnemial crest and a i-idge 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 Iguanodon, Megalosaurus, and 

 many other genera ; but it seems to have become ankylosed 

 in Co in psognathus, Ornithotarsus, and Euskelosaurus. 



The reptiles belonging to this group are for the most part 

 of very large size, and some of them, as the Iguanodon, are 

 among the largest of known terrestrial animals. They 



