Page:Description of New Dinosaurian Reptiles.pdf/4

84 Ornithomimus velox, gen. et sp. nov.

The high degree of specialization in the reptiles above described has a partial parallel in a small group of typical 0rnithopoda from the same horizon. Various specimens of these, recently secured, represent a distinct genus and several species. The most marked characters already determined are manifest in the limbs and feet, and these have been selected for description in the present notice. A typical example is shown on Plate I, figures 1-3, which is the type specimen of the species here described.

On the distal part of the tibia represented in figure 1, the astragalus is seen in place, with a very large ascending process, larger than in any dinosaur hitherto known. The calcaneum is also shown in position, but the slender fibula is absent. This bone was complete, but of little functional value. The tibia and all the larger limb bones were hollow, with thin walls, as indicated in the section, figure 1, c.

In figure 5, the corresponding parts of a young ostrich are shown for comparison. The slender, incomplete fibula is in place beside the tibia. The astragalus with its ascending process, and the distinct calcaneum, are also shown in position. The almost exact correspondence of these different parts in the bird and reptile will be manifest to every anatomist.

The most striking feature of the foot belonging with the reptilian tibia is shown in the metatarsals represented in figure 2, A. These are three in number, and are in the same position as in life. They are the three functional metatarsals of the typical Ornithopoda and of Birds. The distal ends of these bones correspond in size and relative position in the two groups, but here, in the present specimen, the reptilian features cease, and those of typical Birds replace them. In all the reptiles known hitherto, and especially in Dinosaurs, the second, third, and fourth metatarsals are prominent in front, at their proximal ends, and the third is usually the largest and strongest. In birds, the place of the third is taken above by the second and fourth, the third being crowded backward, and very much diminished in size.

This character is well shown in figure 6, which represents the second, third, and fourth metatarsals of a young turkey, with the tarsal bones absent. In the reptilian metatarsals seen in figure 2, the same arrangement is shown, with the tarsals in place. The second and fourth metatarsals have increased much in size in the upper portion, and meet each other in front.