97 



Genus Scelidotherium. 



The following specimens belong to the same skeleton of the Scelidotherium 

 leptocephalum. They were discovered in the cliffs at Bahia Blanca, and were 



Presented by Charles Darwin, Esq., F.R.S. 



486. The skull and right stylo-hyal bone. 



The brain being regulated in its development by laws analogous to 

 those which govern the early perfection of the organ of hearing, appears, 

 from the obvious immaturity of the present specimen, to have been rela- 

 tively larger in the Scelidothere than in the Mylodon : it was certainly 

 relatively longer : the fractured cranium gives six inches of the an- 

 teroposterior diameter of the brain, but the analogy of the Mylodon 

 would lead to the inference that it extended further into the part which 

 is broken away. The greatest transverse diameter of the cranial cavity 

 is four inches eight lines : these dimensions, however, are sufficient to 

 show that the brain was of very small relative size in the Scelidothere ; 

 and, both in this respect and in the relative position of its principal masses, 

 the brain of the extinct Megatherioid closely accords with the general 

 character of this organ in the existing species of the same Order. 



We perceive by the obtuse ridge continued obliquely upwards from 

 above the upper edge of the petrous bone, that the cerebellum has been 

 situated wholly behind the cerebrum : we learn also from the same struc- 

 ture of the enduring parts that these perishable masses were not divided, 

 as in the Manis, by a bony septum, but by a membranous tentorium, as in 

 the Mylodon and Sloths : in the Orycterope there is a strong sharp bony 

 ridge extending into each side of the tentorium. The vertical diameter 

 of the cerebellum and medulla oblongata equals that of the cerebrum, 

 and is two inches three lines ; its antero-posterior extent about one inch 

 and a half. The sculpturing of the internal surface of the cranial cavity 

 bespeaks the high vascularity of the soft parts which it contained, and 

 there are evident indications that the upper and lateral surfaces of the 



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