132 



rior and at the posterior margin of each of the above teeth, as in the 

 upper molar, No. 561. 



564. The symphysis of the lower jaw, with the sockets and roots of the incisive 

 teeth, of the Toxodon platensis. 



Discovered with the foregoing fragments of the lower jaw, at Bahia 

 Blanca, in latitude 39°, on the east coast of South America. 



Presented by Charles Dariv'm, Esq., F.R.S. 



From the remains of the symphysis it will be seen that the jaw was 

 remarkably compressed or narrow from side to side ; while the rami were 

 of considerable depth, in order to give lodgement to the matrices and 

 bases of grinders enjoying uninterrupted growth. The pulps of the six 

 incisors in the lower jaw are arranged in a pretty regular semicircle, 

 whose convexity is downwards ; the teeth themselves are directed forwards 

 and curved upwards like the inferior incisors of the Rodentia. The 

 form and degree of the curvature are shown in the almost perfect 

 incisor (No. 565) r which was found in the same stratum, but belonging 

 to another individual. These incisors are nearly equal in size : they are 

 all hollow at their base, and the indurated mineral substance impacted in 

 their basal cavities well exhibits the form of the vascular pulps which 

 originally occupied them. Sufficient of the tooth itself remains in four 

 of the sockets to show that the. broken incisors, like the nearly perfect 

 one. had only a partial investment of enamel : but though in this respect, 

 as well as in the curvature and perpetual growth, they resemble the 

 ' dentes scalprarii ' of the Rodentia, they differ in having a prismatic- 

 figure, like the inferior incisors of the Sumatran Rhinoceros or the tusks 

 of the Boar. Two of the sides, viz. those forming the anterior convex 

 and mesial surfaces of the incisor, have a coating of enamel about half a 

 line in thickness which terminates at the angles between these and the 

 posterior or concave surface. 



From the relative position of the bases or roots of these incisors, we 

 may infer that they diverged from each other as they advanced forwards 

 in order to bring their broadest cutting surface into line. That they were 

 opposed to teeth of a corresponding structure in the upper jaw, is proved 



