Page:Popular Science Monthly Volume 10.djvu/233

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back-bone, between the two haunch-bones, are united so as to form a solid support upon which the weight of the body rests. Then the thigh-bone becomes very short, and has a back ridge upon its outer articular surface. At the lower end the ridge tits in between the upper extremity of the small bone of the leg, near to the great bone,

PSM V10 D233 Bird dinosaur crocodile leg bone structures.jpg
Fig. 6.—Bird.Dinosaur.Crocodile.

and makes a kind of spring-joint. The small bone of the leg is quite large above, and becomes rudimentary below. It runs out into a style, instead of being long and large, as it is in the case of the crocodile. Then, when you come to the bones of the foot, you find there are no separate bones such as you have here, but the end of the tibia, or large bone of the leg, appears to end in a kind of pulley, a single bone follows the tibia and to the trifurcated extremity of this bone. Upon the extremity of that bone are attached three toes. It is obvious that the contrast between the crocodile's leg on the one hand, and the bird's leg on the other, is very striking. But this interval is completely filled up when you study the character of the hinder extremities in those ancient reptiles which are called the Dinosauria. In some of these the bones of the pelvis, and those of the hind-limb, become extraordinarily similar to those of birds, especially to those of young or foetal birds. Furthermore, in some of these reptiles, the fore-limbs become smaller and smaller, and thus the suspicion naturally arises that they may have assumed the erect position. That view was entertained by Mantel, and was also demonstrated to be probable by your own distinguished anatomist, Leidy, but the discoveries of late years show that in some of these forms the