84 



to thr. postero-intenjal lobe. In the last molar this thickening appears to be 

 developed into the large tubercle back of the second division of the crown. 

 Feeble traces of a basal ridge occupy (he interval of the outer lobes and the 

 • back of the crown. 



Measurements of the lower-jaw specimen oi Microsyojjs gracilis. are as fol- 

 lows : 



Lines. 



Depth of lon-er jaw below last molar 4 



Thickuess of lower jaw below last premolar 2J 



Distance from canine alveolus to back of last molar 10 



Space occupied by the entire molar series , 9| 



Space occupied by tbe premolars 4 



Space occupied by the molars » 5| 



Breadth of first molar 1^ 



Breadth of second molar ., 2 



Breadth of third molar 2 



The specific name of 31. gracilis was originally given under the impression 

 that the remains referred by Professor Marsh to Hyopsodus gracilis pertained 

 to the same animal. A specimen exhibited to the writer by Professor Marsh 

 would indicate that M. gracilis is the same as the animal named by him 

 Limnothcrium elegans. As Microsyops is generically distinct from Limno- 

 therium, as characterized from the typical species L. tyrannus^ the specific 

 name of the former would be Microsyops elegans. 



Another specimen, originally referred to Microsyops gracilis, is represented 

 in Fig. 16, Plate VI, and was found by Dr. Carter near Lodge-Pole Trail, 

 about tea miles from Fort Bridger. It consists of a portion of the left ramus 

 of the lower jaw, containing the j^enultimate molar and part of the last one. 



The only remaining entire molar, a view of the triturating surface of 

 which is given in Fig. 17, closely resembles the corresponding tooth in the 

 specimen first described, except that it is a little larger. (The artist has 

 made it appear different by exaggerating the proportions of the tubercle be- 

 tween the posterior lobes, and leaving it out altogether in the corresponding 

 view of Fig. 15.) The remaining portion of the last molar also agrees with 

 the corresponding portion in the first-described specimen. The lower jaw is 

 comparatively deep, and is nearly straight along the base. The fore part 

 with the symphysis is lost, but it would appear not to have been so shallow 

 and thick as in the former specimen, which leads me to suspect that it 

 perhaps belongs to a different animal. The mental ibramen holds the same 

 relative position as in the other specimen. The ridge bordering the lore 



