THE LIMBS 189 



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 ab- 

 sent. 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 tlie 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. Cen- 

 tralia 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 cal- 

 caneum, to articulate closely with the leg bones. The astragalus of 

 the Theropoda (Fig. 156 b, c, e) fits more or less closely in a depres- 

 sion 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, ap- 

 parently; 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. 



