THE LIMBS 187 



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 rep- 

 tilian 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. ^ In 

 Pariasaurus the centrale and fifth tarsale are not known with cer- 

 tainty. 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 rep- 

 tile. 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 Cotylo- 

 sauria, 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 cal- 

 caneum 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. 1 54 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 condi- 

 tion, 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. Zool. Soc, 1919) reports the presence of three bones in the proxi- 

 mal rov/ of the tarsus of the very primitive Seynwuria, and adopts the view that the 

 true tibiale has disappeared in later reptiles, the astragalus representing the inter- 

 medium only. — Ed.] 



