The mammalian foot, in this respect, is even more primitive than that of the lizards, turtles, and crocodiles, the navicular corresponding to the second centrale, the cuneiforms and cuboid to the four tarsalia. The fourth distale, primitively, as in the carpus and as a general rule in all reptiles, is the largest of the series, corresponding to the greater length and strength of the fourth toe.
The tarsus is known in but two temnospondylous amphibians, both from later rocks than Eosauravus. Trematops (Fig. 151 a), and Archegosaurus. In the former, and according to Baur in the latter also, there are three bones in the proximal row, the tibiale, intermedium, and fibulare; four centralia in the middle row; and five tarsalia in the distal—twelve in all.
Three of these have been lost in all known reptiles, the intermedium, or tibiale, and the third and fourth centralia. Nine bones, then, we may assume was the primitive number of tarsal bones in the reptiles. A separate intermedium has been accredited to certain reptiles, Howesia of the Rhynchocephalia, Oudenodon (Dicynodon) of the Anomodontia, and the ichthyosaurs and plesiosaurs. But, unless such forms have enjoyed an uninterrupted and independent descent of which we have no knowledge from the Amphibia, it is altogether improbable that both intermedium and tibiale have ever been present as separate bones in reptiles since early Pennsylvanian times. Otherwise we must assume that there has been a reversion from the specialized to the generalized condition of the Amphibia in these animals, a seeming impossibility in evolution. Moreover, there are but two bones in the proximal row of the tarsus of the Nothosauria (Fig. 149), and these reptiles are generally supposed to have a real genetic relationship with the plesiosaurs.
There have been various theories as to what has become of the additional bones of the amphibian tarsus.[1] Since Gegenbaur, it is generally believed that the intermedium is fused with the tibiale to form the astragalus. This is denied by Baur, who says there is no evidence of such union. Others have thought that the intermedium alone forms the astragalus, the tibiale represented by the tibial sesamoid, which occurs in certain mammals but is unknown as such in lower animals. In this uncertainty it is better to use the two mam-
- ↑ [For an excellent review of this subject see Broom, 1921, in Proc. Zoöl. Soc., London, pp. 143–155.—Ed.]
