tibia, fibula, and astragalus of such a typical Dinosaurian as Megalosaurus, let us compare these bones with the corresponding bones of Reptiles and Birds, as we have compared the pelvis.
In Reptiles (ordinary Lacertilia and Crocodilia, namely, which are alone at present under consideration),—
1. The proximal end of the tibia has but a very small or quite rudimentary cnemial crest, and it presents no ridge for the fibula on its outer side.
2. The flattened sides of the distal end of the tibia look, the one directly forwards, or forwards and inwards; and the other backwards, or backwards and outwards. And when the posterior edges of the two condyles of the proximal ends of the tibia rest on a flat surface which looks forwards, the long axis of the distal end is either nearly parallel with that surface, or is inclined obliquely from in front and without backwards and inwards.
3. There is no depression in the anterior face of the tibia for the reception of an ascending process of the astragalus.
4. The distal end of the fibula is as large as, or larger than, the proximal end, and articulates largely with a facet on the outer part of the astragalus.
5. The astragalus is not depressed and flattened from above downwards, nor does it send a process upwards in front of the tibia.
6. The astragalus remains quite free from the tibia.
In all these respects any ordinary bird, say a fowl, is very strikingly contrasted with the reptile.
1. The proximal end of the tibia is produced forwards and outwards into an enormous cnemial crest; and, on the outer side, there is a strong ridge for the fibula.
2. When the posterior edges of the condyles of the tibia rest upon a flat surface, the one flat surface of the distal end of the bone looks outwards as well as forwards, and the other inwards as well as backwards, and the 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 reversing 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 ascending process of the astragalus.
4. The distal end of the fibula is a mere style, and does not directly 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 in 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 Emu 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).
Now in every one of these particulars, except perhaps the last, Megalosaurus is far more like a bird than it is like a reptile.
