. Bulletin of the Department of Geology. Geology. 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 interna
. Bulletin of the Department of Geology. Geology. 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 Catonyx 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
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