The hind foot is poorly known in the Therapsida. In Galechirus (Fig. 137 b) of the Dromasauria the fifth tarsale is lost, but a small one has been recognized in the related genus Galesphyrus (Fig. 137 c). The Anomodontia have the astragalus and calcaneum, four tarsalia, and a small, frequently unossified centrale; the fifth tarsale is absent. The tarsus is unknown in other groups.
The tarsus of the modern Sphenodon (Fig. 139 a), unlike the carpus, is highly specialized. In addition to the fused calcaneum and astragalus, the centrale and fifth tarsale have disappeared and the first three tarsalia are fused in the adult.
The tarsus of the Pterosauria (Fig. 155 d), like the carpus, is highly specialized. In the early forms the astragalus is suturally united with the tibia, the calcaneum fused with the astragalus. In the later forms the astragalus is indistinguishably united with the end of the tibia, the calcaneum fused or lost as in birds, forming a large, pulley-like articulation. In the early pterodactyls there were at least three other tarsals; in the later ones, like Pteranodon or Nyctosaurus, there are but two free tarsalia, probably the fourth and the fused second and third, or fused first, second, and third. Centralia are unknown in all.
The tarsus of the dinosaurs (Fig. 156), like the carpus, has been much modified in adaptation to upright-walking habits. There is a tendency in all for the two proximal bones, the astragalus and calcaneum, to articulate closely with the leg bones. The astragalus of the Theropoda (Fig. 156 b, c, e) fits more or less closely in a depression or groove on the under and anterior side of the tibia; in the later forms (e. g., Ornithomimus, Fig. 156 e) developing a high ascending process in front, as in the young of birds—a parallel character which has no genetic value. In the Sauropoda (Fig. 156 i) there is a less close union, perhaps due to the larger amount of cartilage in the joints of these animals. The centrale and first and fifth tarsalia are always absent. The second and third tarsalia are often fused, apparently; the fourth is always single when present. The tarsalia, like the carpalia, are absent in the Trachodontidae (Fig. 156 g); even the fourth is said to be wanting—possibly a vestige yet remains. If really absent it is the only known example among reptiles of the absence of all the bones of the distal row.
