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This tooth was found by itself, imbedded in the banks of the Rio 

 Tercero, or Carcacana, near the Parana, at the distance of a hundred and 

 eighty miles from the locality where the head was discovered. Frag- 

 ments of a molar tooth of a Toxodon, apparently the seventh of the left 

 side, upper jaw, were also found at Bajada de St. Fe, in the province of 

 Entre Rios, distant forty miles from the mouth of the Rio Tercero. 



All the molar teeth are long and curved, and without fangs, as in the 

 herbivorous species of the Rodent order : in those, however, with curved 

 grinders, as the Aperea, or Guinea-pig, the concavity of the upper 

 grinders is directed outward, the fangs of the teeth of the opposite sides 

 diverging as they ascend in the sockets ; but in the Toxodon the con- 

 vexity of the grinders is outward, and the fangs converge and almost 

 meet at the middle line of the palate, forming a series of arches, capable 

 of resisting great pressure. It is this structure which suggested to me 

 the generic term proposed for this extinct Mammal*. 



Of the incisors, the two small ones are situated in the middle of the 

 front of the intermaxillaries, and the two large ones in close contiguity 

 with the small incisors, which they greatly exceed in size. The sockets 

 of the two large incisors extend backwards, in an arched form, preserving 

 a uniform diameter, as far as the commencement of the alveoli of the 

 molar teeth ; the curve which they describe is the segment of a circle ; 

 the position, form, and extent of the sockets are such as are only found 

 in those of the corresponding teeth of the Rodentia among existing 

 Mammalia. 



The matrix, or formative pulp of the large incisors, was lodged, as in 

 the Rodentia, in close contiguity with the sockets of the anterior molars ; 

 and we are enabled to infer, from the form of the socket, notwithstand- 

 ing the absence of the teeth themselves, that the pulp was persistent, and 

 that the growth of these incisors, like those of the Rodentia, continued 

 throughout life. 



This condition, joined with the curvature of the socket, necessarily 

 implies a constant wearing away of the crown of the tooth, by attrition 

 againstopposing incisors of a corresponding structure in the lower jaw: and 



* To£ov, arcus ; oSovs, dens. 

 R 2 



