84 



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

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

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

 back of the crown. 



Measurements of the lower-jaw specimen of Microsyops gracilis are as fol- 

 lows : 



Lines. 



Depth of. lower jaw below last raolar 4 



Thickness of lower jaw below last premolar 2^ 



Distance from canine alveolus to back of last molar 10 



Space occupied by the entire molar series DSf 



Space occupied by the 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 M. 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 

 Limnotherium elegans. As Microsyops is generically distinct from Limno- 

 therium, as characterized from the typical species L. fyrannus, the specific 

 name of the former would, be Microsyops elegans. 



Another specimen, originally referred to Microsyops gracilis, is revjresented 

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

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

 of the lower jaw r , containing the penultimate 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 foramen holds the same 

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



