modern Chelonia (Fig. 154 c); perhaps at other times it is lost. And it is very probable that the first centrale of the amphibian and reptilian tarsus ceased very soon to be ossified, and is not represented, even in a fused condition, in any later reptilian tarsus. It has been shown by Baur and others that the fifth tarsale is not fused with the fourth, but has disappeared.
Among the Cotylosauria there are usually eight tarsal bones.[1] In Pariasaurus the centrale and fifth tarsale are not known with certainty. In the Theromorpha eight are present in all known forms except Ophiacodon (Fig. 152), which has nine. The centrale has not been recognized in the Proganosauria (Fig. 153 a), but there are five tarsalia; until their discovery four were the most known in any reptile. Indeed, Baur based the order Proganosauria chiefly upon this character. All other known reptiles, except certain Therapsida (like the mammals), have not more than seven tarsal bones, the fifth tarsale being invariably absent.
In Pariasaurus, Sclerosaurus, and Telerpeton of the later Cotylosauria, Sphenodon (Fig. 139 a) of the Rhynchocephalia, and most Lacertilia (Fig. 140 d) and Chelonia (Figs. 145 c, 154 d, g), the astragalus and calcaneum are fused into a single bone, and the calcaneum is either fused or lost in the Pterosauria (Fig. 155 d) and some Dinosauria (Fig. 156 i). A free centrale is absent in all modern reptiles, though sometimes suturally fused with the astragalus in the Chelonia (Fig. 154 c).
In the Chelonia the small calcaneum is sometimes free (Fig. 154 c). The centrale is never free. Four tarsalia are usually present, the third sometimes suturally united with the fourth. The fourth tarsale is always large.
In the kionocrane Lacertilia (Fig. 143 b) there is a similar condition, the small calcaneum either indistinguishably fused with the astragalus, or suturally attached in the adult. There is no centrale or fifth tarsale, and the first and second tarsalia are either vestigial or lost. The tarsus of the chameleons (Rhiptoglossa), like the wrist, is very curiously modified (Fig. 143 b). But two bones remain in the
- ↑ [Watson (Proc. Zoöl. Soc., 1919) reports the presence of three bones in the proximal row of the tarsus of the very primitive Seymouria, and adopts the view that the true tibiale has disappeared in later reptiles, the astragalus representing the intermedium only.—Ed.]
