LABIDOSAURUS COPE 



319 



think, the bone has absolutely disap- 

 peared in all modern amniotes, leav- 

 ing a space that was soon occupied 

 by the proximal end of the fifth 

 metatarsal, which came to articulate 

 directly with the large fourth tarsale. 

 In many Permian reptiles a divarica- 

 tion and proximal elongation of the 

 fifth metatarsal had begun. This 

 change in the articulation, it seems 

 to me, will easily account for the 

 change in the form of the fifth meta- 

 tarsal in crawling plantigrade rep- 

 tiles. In the rectigrade, and more 

 ambulatory reptiles, on the other 

 hand, with less functional use of the 

 fifth toe, the metatarsal retained more 

 of its primitive shape and position 

 parallel with the fourth, especially in 

 the forms ancestral to the mammals. 



The chief fallacy in Goodrich's 

 arguments lies in deriving the stego- 

 crotaphous chelonian skull from a 

 single- or double-arched ancestor 

 because of the shape of the fifth 

 metatarsal. It is now conceded that 

 the turtles must have had a direct 

 ancestry from the cotylosaurian type 

 of reptile; if so the hook-shaped 

 metatarsal must have been an inde- 

 pendent acquirement. 



The hands and feet of Labidosau- 

 rus (Fig. 5) are relatively large and 

 powerful, the foot nearly as long as 

 the leg, the hand as the arm. The 

 first digit of each is relatively large 

 and but little divaricated in life; its 



