1917 ] Stock: Structure of the Pes in Mylodon hqrlani 279 



leptocephalum by Burmeister, 20 and his compai'isons with Mylodon 

 and Megatherium, have been of greatest assistance. Reference should 

 also be made to Owen's earlier observations 21 on Scelidotherium. 



COMPAEISON WITH LESTODON 

 The calcaneum in M. harlani differs from that in Lcstodon in its 

 greater constriction below the astragalus, and in the less broadly 

 rounded posterior end. The tendinal groove on the outer border of 

 the calcaneum is much better developed in M. harlani. An important 

 difference between the two genera in the calcaneum, to which attention 

 has been directed by Gervais, but which is not apparent in a com- 

 parison of the figures of the mounted pes of these forms, is the division 

 of the astragalar facet. In this respect Lestodon is more like the 

 Megalonychidae. Gervais also points out that in general shape the 

 calcaneum of Lestodon is more like the corresponding element in 

 Megatherium. In Mylodon harlani the angle between the inner and 

 outer divisions of the tibial articulating surface of the astragalus is 

 smaller than in Lcstodon. Judging from Gervais' figure 22 of the pes 

 of L. armatus, the outer or fibular side of the astragalus is not as deep 

 as in M. harlani. 



The navicular in the Rancho La Brea species is less compressed 

 than in Lcstodon, and the external cuneiform is thicker in the latter. 

 In the latter also the middle cuneiform and the second metatarsal are 

 fused, while in M. harlani these elements are usually separate. Gervais 

 states that the third metatarsal is relatively much longer than in 

 Mylodon. 



In structure and size the second and third digits in the pes of the 

 species from the asphalt are very close to Lestodon. The second digit 

 is composed of two elements, the proximal one of which is a composite 

 structure as in the latter. Digit 3 is composed of three elements ; the 

 two proximal phalanges are as a rule not co-ossified. In Lestodon 



20 Burmeister, H., Bericht iiber ein Skelet von Scelidotherium leptocephalum, 

 Monatsberichte Akad. Berlin, pp. 374-381, 1 pi., 1881. 



21 Owen, E., Description of a considerable part of the skeleton of a large 

 edentate mammal, allied to the Megatherium and Orycteropus, and for which is 

 proposed the name of Scelidotherium leptocephalum. The zoology of the vovage 

 of H. M. S. Beagle, etc., pt. 1, Fossil Mammalia, pp. 73-99, pis. 20-23, pi. 24, 

 fig. 1, pi. 25, pi. 26, figs. 2, 4, and 6, pi. 27, pi. 28, fig. 2, 1840. 



Owen, E., Description of the skeleton of an extinct gigantic sloth, Mylodon 

 robustus, etc., pp. 126-137, 1842. 



Owen, E., On the Megatherium (Megatherium American um, Cuvier and Blu- 

 menbach). Pt. V. — Bones of the posterior extremities, Philos. Trans. Boy. Soc. 

 London, vol. 149, pp. 821-823, 1859. 



22 Gervais, P., op. tit., pi. 7, fig. 6, 1873. 



