86 Annals of the Carnegie Museum. 



on the Sweetwater River, eighteen miles west of "The Devil's Gate 

 in Wyoming." 



In the original description of Merycochcerus rusticus 2 Dr. Leidy 

 gave some of the principal characteristics as revealed by the type. 

 They are the following : The skull is a little more than two thirds the 

 diameter of that of Merycochcerus proprius. The infraorbital arch is 

 deep ; the face has the same abrupt narrowing in front of the orbits as 

 in M. proprius, and the infraorbital foramen occupies a corresponding 

 position. In Oreodon (Merycoidodon) the face narrows more gradually 

 anteriorly, and the infraorbital foramen is situated farther forward. 



Leidy suspected from examining these specimens that the remains 

 from the region of the Niobrara River which he had described under 

 the name of Merychyus medius belong to the same species as the type 

 of Merycochcerus rusticus, and that Merychyus major belonged with 

 Merycochcerus proprius. 



In his Extinct Vertebrate Fauna of the Western Territories (1873), 

 Leidy figured (PI. III.) what is considered as the type of Merycochcerus 

 rusticus, and gave further particulars concerning the small collection 

 from the valley of the Sweetwater River in Wyoming. He here 

 points out differences between the teeth of Merycochcerus and Oreodon 

 (Merycoidodon) which he had not done before. 8 



In Merycochcerus the crowns of the molars are higher than in Ore- 

 odon (Merycoidodon) and when the anterior molar is protruded, the 

 posterior molars, though functional, are partly buried in the jaw and 

 advance as they are worn away. Before the last tooth is fully pro- 

 truded " the anatomical character of the triturating surface of the first 

 molar is totally obliterated, and that of the second molar somewhat 

 destroyed." He also says that the highest points of the crowns of the 

 premolars, especially the upper ones, are in advance of the middle of 

 the crowns, even after they are much worn. In Oreodon (Merycoi- 

 dodon) the highest point is median. 



At this time Leidy was under the impression that Oreodon (Mery- 

 coidodon) and Merycochcerus were distinct though closely related gen- 

 era, the latter from a later geological horizon and the successor by 

 evolution of the former ; but that Merycochcerus is the same as Mery- 

 chyus and from the same geological horizon. Yet in this same discus- 

 sion he gave separate definitions to these genera. 



2 "Remarks on a Collection of Fossils from the Western Territories." Proc. 

 Acad. Nat. Sci. Phila., 1870, pp. I09-IIO. 



3 Leidy's " Extinct Vertebrate Fauna of the Western Territories," p. 199. 



