THE LIMBS 185 



The mammalian foot, in this respect, is even more primitive than 

 that of the Hzards, turtles, and crocodiles, the navicular corres- 

 ponding 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, corre- 

 sponding 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, inter- 

 medium, 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 inter- 

 medium, 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.^ 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 sesa- 

 moid, which occurs in certain mammals but is unknown as such in 

 lower animals. In this uncertainty it is better to use the two mam- 



1 [For an excellent review of this subject see Broom, 192 1, in Proc. Zool. Soc, 

 London, pp. 143-155.— Ed.] 



