1902.] Osborfi, Ajnerican Eocene Primates. 1 97 



(Sp. 13) N. (Thinolestes) anceps Marsh, in the type of which 

 the second lower premolar is bifanged but the paraconid has 

 disappeared upon the second and third molars, which are now 

 truly quadritubercular. The mandibular dentition is other- 

 wise closely similar to that of N. tenebrosus ; the lower jaws 

 are coossified, with the suture visible externally. In the upper 

 molars of this important specimen we find the crown sub- 

 triangular, the primitive triangle with distinct intermediate 

 tubercles, but the hypocone is prominent and well separated; 

 there is also an external intermediate cusp or mesostyle; the 

 first upper premolar is small or rudimentary. The third upper 

 premolar has a broad internal cingulum, the fourth is submo- 

 lariform. 



Second Stage. 



(Sp. 6) N. (Limnotherium) tyrannus Marsh. — (Type, 

 Yale Mus.). The second specimen named in 187 1 was also 

 founded upon a lower jaw. Marsh described this as a "pachy- 

 derm" and distinguished the genus Limnotherium from 

 Notharctus by the single fangs of the first and second lower 

 premolars, by the quadritubercular lower molars "with a 

 rudimentary double tubercle on the anterior margin" (para- 

 conid). This type was probably found upon a somewhat 

 higher level than N. tenebrosus. It marks perhaps the next 

 higher stage of evolution in which the first and second pre- 

 molars have single fangs ; the paraconid is a vestigial tubercle 

 seen on all three lower molars; the third upper molar is- 

 tritubercular. Close to this stage is the type of 



(Sp. 27) N. (Tomitherium) rostratum Cope, with small, 

 spaced, first and second lower premolars; in the latter the 

 fang is still grooved. 



Related to these are the more slender jaws forming the 

 type of 



(Sp. 15) Limnotherium afiine Marsh. In this beautiful 

 specimen, belonging to a young individual, we note a slight 

 progression in the cheek teeth, the fourth upper premolar 

 differs from the first molar only in the absence of the 



