191"] Stock: Structure of the Pes in Mylodon harlani 



275 



similarity to the corresponding phalanx of M. robustus, as interpreted 

 by Owen, is thus shown by the backward projection of the dorsal 

 end of the median ridge. The two flattened articulating surfaces of 

 the proximal end are much less deeply excavated than in the corre- 

 sponding phalanx of digit 2 of the manus. The articulation between 

 the first and second phalanges allowed but little vertical movement. 

 The external convexity of the distal trochlea has a greater depth than 

 the internal, and its articulating surface is flatter. A certain amount 

 of compensation for the outward twisting of metatarsal 3 is obtained 

 through the articulations of the first and second phalanges, which tend 

 to allow the ungual to play in a more nearly vertical direction. 



Figs. 7a and 7b. Mylodon harlani Owen. First and second phalanges, third 

 digit of pes, no. 22769, X %. Kancho La Brea beds. Fig. 7a, superior view; 

 fig. 7b, outer view. 



Figs. 8a and 8b. Mylodon harlani Owen. Co-ossified first and second pha- 

 langes, third digit of pes, no. 22770, X %. Bancho La Brea beds. Fig. 8a, 

 superior view; fig. 8b, outer view. 



Among the materials representing digit 3 of the pes of Mylodon 

 harlani there are several specimens which were evidently formed by 

 co-ossification of first and second phalanges (compare figs, la, lb with 

 8a, 8b). The fused element is always somewhat shortened proximo- 

 distally. A similar co-ossification occurs in the third digit, pes, of 

 Megatherium, Megalonyx, and Nothrotherium, though it is worthy 

 of note that in the Miocene genus Hapalops the two phalanges are 

 separate. Among the Mylodontidae the same structure obtains in 

 C atony x and Scelidotherium, but the co-ossified element is broader 

 and more fore-shortened than the Rancho La Brea specimens. The 

 two phalanges are separate in Lestodon, Pseudolestodon, 10 and in 

 Mylodon robustus. The lack of any great movement between the 



io Bautenberg, M., Ueber Pseudolestodon hexaspondylus, Palaeontographica, 

 Bd. 53, s. 1-50, 6 taf., 1906. This genus is considered by Lydekker as syn- 

 onymous with Mylodon, by other authors with Lestodon. 



7b 



7a 



8a 



8b 



