The tibia is about three-fourths the length of the femur, very stocky and heavily built. On the proximal end, the convex external condyle is much narrower antero-posteriorly than the larger and slightly concave internal condyle. The low spine is bifid. A cnemidial crest extends to the middle of the bone. On the distal end, the broad and shallow external articular facet is separated from the narrow and deeper internal facet by a low intercondylar ridge. The fibula is fused to the tibia at the upper end, but is free below, being approximated to the tibia along a rugose surface nearly an inch long. This bone is rather slender and strongly bowed outward. Distally, there is a large facet for the outside of the astragulus, the back part of which rests on the calcaneum. This is peculiarly developed so that the articulation represents what is two separate facets, the one for the outside of the astragulus the other for the calcaneum. Here, however, they are blended. [Illustration: Fig. 19. Right foot, the phalanges in outline from the left foot—½ natural size.] [Illustration: Fig. 20. Astragulus plantar side—_a_, external facet; _b_, sustentacular facet—½ natural size.] While in general the tarsus is similar to that of _Theosodon_, there are some marked contrasts. The astragulus has an asymetrical trochlea with a shallow groove, the external condyle being higher and narrower than the internal. The head is depressed in the dorso-plantar plane, is carried on a moderately long neck, and has a broad convex facet for the navicular on which alone it articulates. On the plantar side, the ectal facet is broadly oval and slightly concave, differing from that of _Theosodon_ in having no sulcus dividing it into lobes. The broad sustentacular facet is slightly convex, and widely separated from the ectal. On the external side the astragulus carries an expanded articular facet for the inner side of the fibula, which, instead of being vertical, is expanded below, making an oblique face which is continuous with the fibular facet on the calcaneum. In this feature _Protheosodon_ is, as far as the feet are known, unique. The calcaneum is a long bone with a club-shaped expansion of the upper end. The fibular facet is small, being continuous, as above described, with that on the outer side of the astragulus. On the face toward the astragulus, the ectal facet is broadly convex (not divided as in _Theosodon_), while the sustentacular facet is slightly concave. The distal end is occupied by the large concave facet for the cuboid. The navicular is of moderate height, with a prominent hook behind. On the upper surface is only the broad, deep facet for the astragulus; while the lower surface is divided into facets for the three cuneiforms, and the external distal margin is beveled to make a narrow facet for the cuboid. This navicular differs from that of Theosodon in that the facet for the ectocuneiform is not cut step-like into its external face.