DAKOTA AND NEBRASKA. 



131 



Antero-post. diameter first upper premolar, 

 Length of lower molar series, 

 Length of lower true molar series, 

 Antero-post. diameter last lower molar, 

 Antero-post. diameter second lower molar, 

 Antero-post. diameter first lower molar, 

 Antero-post. diameter last lower premolar, 

 Ant.-post. diameter second lower premolar, 

 Anlero-post. diameter first lower premolar. 



AGRIOCHCERID^. 



A peculiar and extinct family of ruminants of the most aberrant character, but 

 allied to the Oreodonts, is indicated by the remains of a single genus, — Agriochwrus, — of 

 which three species appear to be distinguishable. The remains were obtained from 

 bed B of Dr. Hayden's section of tlie miocene formation, on the Mauvaises Terres of 

 White River, and they are of comparatively rare occurrence. 



The principal distinctive features of the family as seen in the skull oi Agriochcerus, 

 are briefly as follow : The skull has the general form and construction as in 

 Oreodonts, but the orbits are open behind as in the representative of the ruminants of 

 the early tertiary period, the Anoplotherium. No lachrymal fossas exist in front of 

 the orbits. The formula of dentition is the same as in the Oreodonts. The true 

 molars, though constructed after the ruminant type, are remarkable for their trans- 

 versely spreading character, or the shallowness and breadth of their crowns. The 

 fourth upper premolar departs from the usual ruminant character in the possession of 

 three demiconoidal lobes to the crown, two externally and one internally. The third 

 lower premolar is nearly like the succeeding pair of true molars. 



The two extinct families of the Oreodonts and Agriochcvrids which I have attempted 

 to characterize evidently hold an intermediate position between the ruminants and 

 suilline pachyderms. Notwithstanding their equally close relationship with the 

 latter, I have preferred classifying them with the former, on account of the construc- 

 tion of their true molar teeth, so eminently characteristic among living animals of 

 that remarkable habit, the rumination of food. 



