84 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



being reduced to mere nodular vestiges of the proximal ends, and the 

 metapodials are longer and more slender than in any other genus of the 

 family. 



Dentition. — {V\. XIII, figs. 14-18.) 



A. Upper Jaw. The upper incisor, though caniniform, is notably smaller 

 than in Proterotherium and is somewhat more rounded and less trihedral in 

 cross-section. Though young skulls with the milk-teeth are not uncommon, 

 I have seen no skull with unworn premolars. When considerably worn 

 the last three premolars have a narrow, triangular valley in the middle of 

 the crown. While the elements of the molar crown are the same as in 

 Proterotherium, yet the appearance is quite different ; in m- and m- the 

 anterior conule and the two internal cusps are connected by a continuous 

 and somewhat longitudinal ridge, the various cusps projecting but slightly 

 below the level of the crest and demarcated by very shallow vertical 

 grooves. The apices of the cusps are very early worn away and then the 

 tooth has merely a central enamel valley like that of the premolars. The 

 posterior conule is greatly reduced and forms but a narrow and slender 

 spur from the longitudinal ridge, which may or may not reach the outer 

 wall of the crown. The third molar differs from the other two in the 

 great reduction of the postero-external cusp, which is convex and more 

 posterior than external in position ; the postero-internal cusp is very small 

 and separate from the antero-internal. 



B. Lower Jaw. One of the characteristic features of Thoatherium is to 

 be found in the relative size of the two lower incisors, the lateral being very 

 little larger than the median, a marked distinction from all the other 

 genera of the family. Both of the incisors are more procumbent than in 

 Proterotherium, pointing almost directly forward. The canine and first 

 premolar resemble those of the last named genus, but the second differs 

 in the internal ridges, of which the anterior one is smaller and the pos- 

 terior one larger. The third and fourth premolars are molariform. The 

 molars resemble those of Diadiaphorus rather than those of Proterothe- 

 rium, as they have no postero-internal pillar and m-g- has no talon, while 

 the external angles of the crescents are more rounded and not so sharp. 



Milk-TeetJi. — The upper deciduous incisor is somewhat smaller than its 

 successor and of rather more trihedral shape. After a short diastema this 

 tooth is followed in some individuals, not in all, by a second, very small, 

 premaxillary tooth, probably di-, which is acutely conical in shape. The 



