1 98 Bulleti7i American Museum of Natural History. [Vol. XVI, 



hypocone, and sexituberculy is slightly more marked in the 

 true molars than in N . anceps; the first and second lower pre- 

 molars are single fanged, and, correlated with the development 

 of the hypocone above, the paraconid has degenerated and 

 disappeared upon the first and second molars below. 



(Sp. 7) N. (Limnotherium) elegans, according to Marsh, is 

 a much smaller but related species. 



(Sp. 5) N. (Hyopsodus) gracilis Marsh exhibits a paraconid 

 on the lower molars and thus either antedates Notharctus 

 {Limnotherium) elegans or is possibly referable to Sarcolemur. 



Third Stage. 



It is evident from the study of the foregoing series that 

 this line would inevitably terminate in sexituberculy above 

 a,nd quadrituberculy below. This condition is fulfilled in the 

 type of 



(Sp. 14) N. (Telmatolestes) crassus Marsh, a large species 

 in which the first and second upper molars are nearly quad- 

 rate in form and bear six tubercles, the hypocone being almost 

 as large as the protocone on m^ and m^ ; the hypocone how- 

 ever is wanting on m^. The lower molars still retain a 

 faintly developed paraconid. Near this stage is 



(Sp. 10) Hipposyus formosus Leidy, founded upon a single 

 tipper molar (Leidy, '73, plate vi, fig. 41). 



General Characteristics of the Teeth. — Notharctus was very 

 abundant in the Bridger period and as seen in the above 

 analysis presented progressive variations which are certainly 

 due to the passage from lower to higher geological levels. 

 The two pairs of incisors are compactly placed, with 

 chisel-edges as contrasted with the rounded incisors of 

 Pelycodus; the opposite pairs are spaced, that is there is a 

 slight interval between them. The unworn lower molars are 

 elongate (Fig. 23); they exhibit a transverse anterior crest 

 (metalophid) between the protoconid and metaconid, in front 

 of which is an oval valley bounded internally by the para- 

 conid in all stages of degeneration; behind this ridge is the 



