83 



In the genus Limnothoriuni, as established by Professor Marsh, from the 

 typical species Z. ti/nmnus, the dental formula consists of two incisors, a 

 canine and seven molars. 



Mycrosyops gracilis. 



The more characteristic specimen upon which this species was named con- 

 sists of a portion of the left ramus of the lower jaw, represented in Fig. 14, 

 Plate VI, of the natural size. The specimen was discovered by Dr. Carter 

 on Grizzly Butte. Besides the fang of the canine and those of the premolars, 

 it contains the true molars entire. These are moderately worn at the sum- 

 mits of the constituent lobes of their crowns, and their triturating surfaces 

 are represented in Fig, 15 of the same plate, magnified four diameters. The 

 jaw-fragraent retains part of the rough sutural surface of the symphysis, show- 

 ing that its union was ligamentous with the other ramus. The basal portion 

 beneath the molars is broken away. Below the premolars it is of moderate 

 depth and thickness, and soon curves upward with the fang of the canine. The 

 mental foramen is situated below the second premolar. The fang of the 

 canine indicates a proportionately larger tooth than in Hyopsodus. It is lat- 

 erally compressed and curves ujjward, forward, and t)utward. The transverse 

 section is oval with the long diameter directed obliquely forward and out- 

 ward, and measuring 1.8 line, while the short diameter is 1 line. 



The premolars successively increase in size. The first is separated by a 

 slight hiatus from the canine, and was inserted by a single fang. The others 

 have two fangs. 



The molars have oblong quadrately-oval crowns of nearly uniform size. 

 They are inserted with two fangs, with the back one of the last molar widened, 

 as usual in most ungulates. 



The crown of the molars is composed of two divisions, in addition to 

 which the last one has a large posterior tubercle. The anterior division of the 

 crown is smaller than the posterior, and appears of the same form in a more 

 contracted condition. Each division consists of an external crescentoid coni- 

 cal lobe and an internal rudimental conical lobe or tubercle, whicli is placed 

 opposite the back part of the former lobe. 



The front arm of the anterior crescentoid. lobe ends in a thickening in ad- 

 vance of the antero-internal lobe. Tlie corresponding arm of the better de- 

 veloped posterior crescentoid lobe terminates at the base of the lobe in front 

 of it. The back arm of the same lobe forms a slight thickening contiguous 



