205 



teeth are worn away. The differences mentioned — that is to say, the back- 

 ward inclination of the crowns of the premolars and the more advanced 

 position of their points in Merycochoerus — would appear to be due to a com- 

 parative shortening of the face and a less consequent space for the develop- 

 ment of the teeth. 



The same differences which have been mentioned as existing between the 

 premolars of Merycochcerus rusticus and Oreodon are also obvious in Mery- 

 chyus elegans. The same may be said also of the third upper premolar in 

 the fossil referred to Merychyus major, except that in this the crown of the 

 tooth is proportionately not so long as in Merycochcerus rusticus, and was less 

 worn when fully protruded. . 



In Merycochcerus rusticus the outer face of the upper premolars is convex 

 longitudinally, but concave transversely; the lateral borders having a consider- 

 able degree of prominence. In M. proprius and Merychyus major they are like- 

 wise concave and bordered by a strong basal ridge which is absent in Mery- ■ 

 cochcerus rusticus. In Merychyus elegans the outer face of the upper pre- 

 molars is convex transversely as well as longitudinally, and, as in the latter, is 

 devoid of a basal ridge. 



In a small fragment of an upper jaw of M. rusticus, containing the second 

 and third premolars, represented in Figs. 3, 4, Plate VII, the crowns are com- 

 paratively but little worn and retain the characters of the triturating surface. 

 These teeth are of less breadth in proportion to their thickness than in M. 

 proprius, and in this respect are more like the corresponding teeth of Oreodon. 

 Their outer part forms a strong curve from the ends of the fangs to the point 

 of the crown, of which about one-fourth externally remains unprotruded, 

 while it is fully protruded internally. The point of the crown is at the 

 anterior third, and externally it appears to be continuous as part of the an- 

 terior projecting border of the crown. The inner portion of the crown ex- 

 hibits three deep recesses inclosed by prominent loop-like folds. The pos- 

 terior larger recess is separated from the anterior smaller pair by a ridge 

 dividing the inner part of the outer or principal lobe of the crown. A basal 

 ridge festoons the posterior internal loop of the third premolar, but does not 

 exist in the second. The teeth are worn off in a slope on the postero-internal 

 face of the principal lobe of the crown. 



