The navicular is broad and low, with a prominent hook behind. On its upper face there is only the broad facet for the astragulus head; on the lower face are three facets, externally, a large, more or less triangular area, for the ectocuneiform; medianly a smaller similar facet for the mesocuneiform; and on the internal side, sloping up onto the internal face, a small facet for the reduced endocuneiform. On the external face of this bone there is a tiny beveled facet for the cuboid.
The endocuneiform is a large scale-like ossicle articulating on the lateral internal face of the navicular, and overlapping markedly the inner surface of Metatarsus II.
The mesocuneiform is considerably reduced in size, carrying a broad flat facet on the upper surface for the navicular, and a shallow saddle-like one below for Mt. II, which is entirely carried by this bone.
The ectocuneiform is considerably larger than the mesocuneiform, resting above on the navicular, and carrying below the whole of Mt. III. On its inner side are two facets which rub against the upper end of Mt. II.
The cuboid is a nodular bone, its upper surface occupied by the facet for the calcaneum, the lower face occupied by the facet for Mt. IV, while on the external side there is a tiny beveled facet for the vestige of Mt. V, and with a small boss on the inner surface which carries two tiny facets, the upper one for the ectocuneiform and the lower for the navicular. On this same inner side, near the top there is a second small boss, which carries a tiny facet to rub on the astragulus, and below that a second tiny facet for the navicular.
Fig. 12. Cuboid internal side
to show:
a, facet for astragulus;
b, upper facet for navicular;
c, facet for calcaveum;
d, lower facet for navicular;
e, facet for mesocunieforms
—½ natural size.
The pes consists of three digits, with a vestige of Mt. V. Of the developed digits, the median one is the largest, but the two lateral digits are only a little smaller and were functional, so that this form was truly three-toed, comparable in the digital reduction to Mesohippus.
Mt. II is flattened above but soon broadens into a rounded shaft of considerable length, on the end of which is the articular trochlea, with the carina extending onto both the upper and lower surface, being, however, higher on the lower surface. Proximally this bone is overlapped by the endocuneiform, is carried by the small mesocuneiform, and also articulates on the inner side of the ectocuneiform. Mt. III is also compressed at the upper end, broadens below, and carries an articular trochlea similar to that of Mt. II, except that the carina does not extend so far onto the upper surface. Like Mt. II, Mt. IV is carried high on the tarsus, and therefore, though nearly as long as Mt. III, it does not have the same effective length. Proximally it articulates entirely on the cuboid; distally it has a trochlea similar to that of Mt. II, the carina extending onto the dorsal surface. While Mt. V is lacking, it is clearly indicated that a vestige of it should have been present, as there is a tiny articular surface for it on the cuboid, and a rugose surface on the outside of Mt. IV.
The phalanges are long and have the articular ends swollen somewhat as in camels. The phalanges of the first row are nearly equal in size, each with the proximal trochlea deeply notched for the carina of the metatarsus; and with the distal trochlea simple, though slightly concave from side to side, and reflexed well onto the dorsal surface. The phalanges of the second row are shorter and simpler, and somewhat depressed distally. The ungual phalanges are flattened from top to bottom, of moderate size, somewhat longer than wide, and without any indications of a cleft.