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VII. On the Scelidothere (Scelidotherium leptocephalum, Owen). 
By Professor Owen, F.B.S. &c. 
Eeceived October 30, — Eead December 18, 1856. 
The extinct species of large terrestrial Sloth, indicated by the above name, was first 
made known by portions of its fossil skeleton discovered by Charles Darwin, Esq., F.E.S., 
at Punta Alta, Northern Patagonia, which were described by me in the chapters of the ' 
Appendix to the ‘Natural History of the Voyage of H.M.S. Beagle,’ treating of the 
“ Fossil Mammaha* ” collected during that voyage. 
The subsequent acquisition by the British Museum of the collection of Fossil Mam- 
maha brought horn the pleistocene beds, Buenos Ayres, by M. Bravard, has given 
further evidence of the generic distinction of the Scelidothere from the other Gravi- 
grades of the Bruta phyUophaga\, and has supplied important characters of the osseous 
system, and especially of the skull, which the fragments from the hard consohdated 
gravel of Punta Alta did not afibrd. The best portion of the cranium from the latter 
locahty wanted the facial part anterior to the orbit, and the greater part of the upper 
walls ; sufiicient, however, remained to indicate the peculiar character of its slender pro- 
portions, and hence the name leptocephalum proposed for the species. 
The aptness of the epithet ‘ slender-headed ’ proves to be greater than could have been 
surmised trom the original fossil ; for the enthe skull now in the British Museum (Plates 
VIII. and IX.) exhibits a prolongation of the upper and lower jaws, and a slenderness of 
the parts produced anterior to the dental series, unique in the leaf-eating section of the 
Order Bruta, and offering a very interesting approximation to the peculiar proportions of 
the skull in the Ant-eaters. 
The original fossils on which the genus Scelidotherium was founded, indicated, by the 
persistence of the cranial sutures and the condition of the epiphysial extremities of the 
long bones, that they had belonged to an individual of immature age : the difference of 
size between them and the corresponding parts in the British Museum, depends on 
the latter having belonged to full-grown individuals : the slight difference in the shape 
of the anterior molars seems, in like manner, to be due to such an amount of change as 
might take place in the progress of growth of a tooth with a constantly renewable pulp. 
I find, at least, no good grounds for inferring a specific distinction between the mature, 
if not old, Scehdothere from Buenos Ayi’es and the younger specimen from Patagonia. 
♦ 4to, 1838-40. 
t See ‘ Conspectus ‘Familiarum et Generum Brutorum frondes carpentium,’ in ‘Description of the Skeleton 
of the Mylodon robusttis,' 4to, 1842. 
MDCCCLVII. 
P 
