1917] 



Stock: Structure of the Pes in Mylodon harlani 



281 



metatarsal is considered to be fused, and metatarsal 2 are present as 

 separate elements. It is probable, therefore, that in Burmeister's 

 figure the fused element represents more than the internal cuneiform 

 and the remnant of digit 2. In Mylodon liarlani the internal cunei- 

 form has not been observed and from all appearances seems to have 

 disappeared with the first digit. The additional loss of a functional 

 second digit in SceUdotherium indicates a greater specialization in the 

 pes than in M. liarlani. 



In discussing the relation of the cuneiforms to the adjoining meta- 

 tarsals Burmeister states : "... bei Mylodon nur ein einziges, grosses 

 cuneiforme sich findet, an welches die Metatarsusknochen von drei 

 Zehen (I, II, III) stossen." 28 This is certainly not the case in M. 

 harlani and, as shown by Owen, in M. robustus also, where the external 

 and middle cuneiforms are separate elements supporting the third and 

 second metatarsal respectively. That peculiar fusion of various 

 bones of the pes is apt to occur is well shown in one of the skeletons 

 of Mylodon robustus in the American Museum of Natural History, in 

 which, according to Dr. Matthew, the external cuneiform is co-ossified 

 with the third metatarsal. 



In the marked widening and shortening of the third metatarsal 

 and the presence of a heavy proximal segment in the digit which 

 supports distally a very large ungual, SceUdotherium is decidedly 

 different from M. harlani and is strongly suggestive of the Pleistocene 

 Megalonychidae. The large ungual in SceUdotherium is supported by 

 the co-ossified first and second phalanges, while in Mylodon the cor- 

 responding ungual is much smaller and is supported by first and 

 second phalanges, which are usually separate. In the slender fourth 

 metatarsal of the former genus the articulating faces for cuboid and 

 metatarsal 3 are separated by a distinct angle, while in M. harlani 

 they are continuous in the same plane. In SceUdotherium the three 

 phalanges of digit 4 are represented. The proximal and middle 

 elements have co-ossified, while the distal or terminal phalanx is rudi- 

 mentary. In M. harlani only two segments of this digit are present, 

 but the middle or second phalanx has not become fused with the 

 proximal element. The reduction of the fourth digit in SceUdotherium 

 is, therefore, not as advanced as in Mylodon. Burmeister believes 

 that a closer agreement of SceUdotherium with Megatherium than with 

 Mylodon is shown in that "die beiden Metatarsusknochen der zwei 

 ausseren Zehen nur am cuboideum gelenken, und nicht, wie bei 



28 Burmeister, H., op. cit., p. 380. 



