DAKOTA AND NEBRASKA. 149 



of the premolars it is about as thick as in the latter, but still holds the same relative 

 depth as beneath the premolars. 



Internally the alveolar portion of tlie jaw is as convex posteriorly as in the Camel, 

 but is much less so anteriorly. Below the alveolar portion, along the position of the 

 base, the bone is depressed, especially at the back part. The portion of the jaw 

 accommodating the premolars, in comparison with that in the Camel, is much 

 thinner. 



The base of the jaw is nearly as thick as in the Camel, but its line approaching the 

 symphysis is more sigmoid. In advance of the premolars the base retains its thick- 

 ness, but the bone above is thinner and rises towards the hiatus of the teeth in a more 

 carinate manner, as in the Lama. 



The symphysial portion of the jaw appears to have been much shorter than in the 

 Camel. The symphysis terminates posteriorly below the position of the crown of the 

 caniniforra pi-emolar, whereas in the former animal it reaches for back of this position. 



The mental foramen occupies a similar position to that in the Camel, — that is, below 

 the caniniform premolar. A second foramen occupies a position not quite so far back 

 as its homologue in the Camel and Lama, being situated below the last premolar, 

 instead of the first true molar as in those animals. 



As before intimated, the molar series, though composed of smaller teeth than in 

 the Camel, from their greater number occupies a greater extent of space along the 

 alveolar border of the jaw. Thus in a lower jaw of the Camel, in which the four 

 molar teeth occupy a space of five inches and a half, in the fossil the six teeth occupy 

 a space of six inches and a quarter. 



The true molars, as seen in figures 1, 2, e, g, plate XV, are almost the counterpart 

 of those of the Camel. In tlie first one, in the fossil, the crescentoid intervals, which 

 separate the inner and outer lobes in unworn teeth, are completely obliterated, leav- 

 ing a yoke-shaped triturating surface of dentine bordered by enamel internally and 

 externally, as represented in figure 2, e. 



The last molar yet retains the crescentic pits intervening between the anterior 

 pairs of lobes. The pits appear to be as contracted and their sides as vertical as in 

 the Camel. The worn triturating surfiice of this tooth, represented the size of nature 

 in figure 2, g, is almost an exact likeness of its homologue in the latter animal at the 

 same stage of abrasion. The tooth exhibits a single important difference from that 

 of the Camel in the greater degree of distinction of the internal lobes internally 

 througli a more strongly folded condition of their posterior border. 



The fourth or last premolar, figures 1, 2, d, has the crown much worn, nevertheless 

 it appears to have been proportionately shorter, about as wide and thinner than in 

 the Camel, but has nearly the same form. The triturating surface presents a minute 

 islet at its back part, the remains of the pit seen in a similar position in the less worn 

 tooth of the Camel or Lama. 



