OF THE WHITE RIVER OLIGOCENE. 55 



roots of pi, which are some 4 mm. removed from the canine alveolus 

 and 14 mm. from the alveolus for the anterior root of p„. In the 

 upper dentition, where there is no indication of premolar crowding, 

 a very decided simplification in the crown pattern of the third molar 

 is found and a reduction in the size of the tooth as compared with 

 other specimens in which a larger number of cusps are present. In 

 the anterior row (Fig. 3 A, B, left m^) there are two large cusps 

 slightly joined in front by a sharp saddle, with almost no protoconule 

 (Fig. 3 5) or, in another specimen, with a small development of this 

 cusp (Fig. 3 ^). In the posterior row, the metacone is well de- 

 veloped, there is a very small metaconule sometimes not completely 

 differentiated from the former, and a heavy posterior cingulum, 

 which terminates against the protocone without any suggestion of a 

 hypocone. The second Princeton specimen just referred to (No. 

 12544) showing this type of third molar is also from the lower zone 



Fig. 2. Archceotherium mortoni, No. 12624 Princeton University Geologi- 

 cal Museum. Right ramus of lower jaw from the outer side, about one fifth 

 natural size. The anterior teeth are indicated by dotted outlines to localize 

 the alveoles. 



of rusty nodules of the Lower Oreodon beds, but lacks the lower jaw. 

 In the collection of the American Museum of Natural History a very 

 small skull, No. 1481, from the Oreodon beds on Hat Creek, Nebraska, 

 shows an extreme degree of crowding of the lower anterior premolars 

 (c-pi, 4 to 5 mm. ; pi-po, 2.5 mm. to 3 mm. ; Po-Ps, 3 mm. to contact) 

 associated with a type of m^ indistinguishable from that just de- 

 scribed. In a partial skull without lower jaws in the same collection, 

 from the Middle Titanotherium beds on Lance Creek, Wyoming, the 

 third upper molar (Fig. 5), which here has a slight external cingulum, 

 shows even greater reduction of the cuspidation in the posterior row 

 where the posterior intermediate is absent and metacone and posterior 

 cingulum blend. 



