95 



inner surfaces encroach so far upon the substance of the tooth, that the 

 central coarse or vascular dentine is, as it were, squeezed out of the in- 

 terspace, and the elevated ridge of the dense ivory describes an hour-glass 

 figure upon the triturating surface, the connecting isthmus being but 

 half the breadth of the rest of the tract: the external cement preserves 

 nearly an equal thickness throughout. Of the two lobes into which 

 this tooth is divided by the transverse constriction the anterior is the 

 largest ; their proportions and oblique position are pretty accurately 

 given in the figure. 



From the cliffs at Bahia Blanca, near Patagonia. 



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



47G. The third molar tooth, left side, lower jaw, of the Mylodon Harlani. 



Purchased. 



477- A portion of the atlas of a Mylodon, probably Myl. Darwinii: the 

 transverse processes are wanting ; the canal for the second pair of spinal 

 nerves is converted into a foramen by a bridge of bone connecting the 

 posterior margin of the neural arch with the posterior oblique process. 

 From the tertiary deposits of the Pampas of Buenos Ayres. 



Purchased. 



478. The body and one of the cornua majora of the os hyoides of a Mega- 

 therioid animal equal to the Mylodon in size ; probably the Mylodon 

 Darwinii. 



From the tertiary deposits of the Pampas of Buenos Ayres. 



Purchased. 



479. The cerato-hyal bone of, probably, the Mylodon Darwinii. 



From the ternary deposits of the Pampas of Buenos Ayres. 



Purchased. 



480. The smaller or hyoid moiety of the stylo-hyal bone of, probably, the 



same animal. 



From the tertiary deposits of the Pampas of Buenos Ayres. 



Purchased. 



481. The manubrium sterni of a Mylodon, probably Myl. Darwinii. 



The two anterior lateral margins are thinner and slightly concave ; 



