94 



longer, but is inclined forwards at a more open angle with the horizontal 

 ramus : the two tuberosities on the outside of the symphysis for the 

 attachment of the retractors of the lower lip, are much more strongly 

 developed in the Mylodon Darwinii than in the Mylodon robustus. 

 From the cliffs at Bahia Blanca, near Patagonia. 



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



473. A section of a portion of the left ramus of the lower jaw of the Mylodon 



Darwinii, showing the forms of the transverse sections of the teeth and 

 the depth of their implantation in the alveoli. The outer part of the 

 hard dentine is black, as if carbonized or charred ; the rest of the tooth 

 retains its light colour. The first molar in the present jaw is the 

 smallest and simplest of the series : its transverse section is ellipsoid or 

 subovate, narrowest in front, and somewhat more convex on the outer 

 than on the inner side. The second tooth presents in transverse section 

 a more irregular and wider oval figure than the first ; the line of the 

 outer side is convex, but that of the inner side is slightly concave in con- 

 sequence of the tooth being traversed longitudinally by a broad and 

 shallow channel or impression. 



From the cliffs at Bahia Blanca, near Patagonia. 



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



474. A transverse section of the third molar tooth, left side, lower jaw. 



The transverse section of this tooth has a trapezoidal or rhomboidal 

 form : the angles are rounded off: the posterior one is most produced : 

 the anterior and posterior surfaces are flattened, the latter slightly con- 

 cave in the middle : the external and internal sides are concave in the 

 middle, especially the inner side, where the concavity approaches to the 

 form of an entering notch. 



From the cliffs at Bahia Blanca, near Patagonia. 



Presented by Charles Da?'ivin, Esq., F.R.S. 



475. A transverse section of the fourth and last molar tooth, left side, lower jaw. 



The last molar, which is generally the most characteristic in the 

 fossil Bruta, presents in an exaggerated degree the peculiarities of 

 the preceding tooth : the longitudinal channels on both the outer and 



