MEGATHER1ID.E. 91 



the Pleistocene of Buenos Ayres, Argentine Republic. 

 This specimen, of which the transverse diameter is only 

 0,110, differs from the corresponding bone of M. ameri- 

 canum by the higher position of the navicular cup, which 

 is also narrower ; in these respects the present specimen 

 is intermediate between M. americanum and Mylodon, in 

 which the cup is absent. 



Presented by Senor L. J. Fontana, 1871. 



Genus SCELIDOTHERIUM, Owen \ 

 Including Platyonyx, Lund 2 . 



According to Burmeister (' Monatsb. k. preuss. Ak. Wiss. ' 1881, 

 pp. 374-380) Platyonyx is distinguished by the structure of the 

 foot and the absence of an entepicondylar foramen to the humerus ; 

 but since the cranium of S. chiliense (infra, p. 100) agrees with that 

 of typical species of Platyonyx, whilst its humerus has an entepi- 

 condylar foramen, it appears either that the limb-bones referred to the 

 latter belong to some other genus 3 , or that the characters on which 

 that genus is founded can only be regarded as of specific value 4 . 



The genus is in some respects intermediate between Megatherium 

 and Mylodon, approaching the former in the structure of the cranium 

 and pcs, and the latter in dentition. S. leptocephalum is the most 

 aberrant form, and shows signs of affinity with the Myrmecophayidce. 



The teeth are j in number and have no diastema ; the upper ones 

 have an irregular oval section and are set obliquely in the jaw, the 

 last being the smallest ; the first three lower ones are triangular, 

 and the fourth elongated with an inner crotchet, which varies in 

 size in different species. The cranium is long, narrow, and low, the 

 inferior border of the mandible being frequently nearly straight, 

 although in some species having a descending hinder portion, recalling 

 the jaw of Megatherium. The astragalus wants the articular navi- 

 cular cup characteristic of Megatherium, and varies considerably in 

 form in the different species, that of the type species being the 

 'most unlike that of the latter genus. There are four digits to the 

 manus 5 , the second and third being provided with nearly straight 



1 Zoology of the Voyage of the ' Beagle,' pt. i. p. 73 (1840). 



2 K Danske Vid. Selsk. Skr. vol. ix. p. 145 (1842). Separate copies of the 

 memoir are dated earlier. 



3 Owen (' Memoir on the Mylodon' p. 170, note) suggests that the limb- 

 bones referred by Lund to Platyonyx may belong to Glyptodon. 



4 See Lydekker, Proc. Zool. Soc. 1886, p. 496. 



5 The feet are figured by Burmeister in the ' Monatsb. k. preuss. Ak. Wiss.' 

 1881, plate facing p. 380. 



