1884.1 07l lCope 



GENERAL CONCLUSIONS. 



From what is now known of the history of the Oreodontidse, the following 

 conclusions may be drawn. These are especially instructive as far as 

 they go, since they involve the causes of the rise, great development, de- 

 cadence and extinction of one of the best-marked types of Mammalia the 

 world has seen. The history of this type involves more or less the history 

 of the life of the North American continent during the Miocene epoch of 

 Tertiary time. It moreover involves the laws which regulate the vital 

 success of all types of life, and which express the causes of multiplication, 

 of energy, of weakness, and of sterility. 



Two lines of the family, the Oreodontince and the Agrioclmrince, come to 

 light simultaneously in geological time, the White River epoch, or the 

 Oligocene. The latter is a higher type than the former in its more com- 

 plex fourth premolars, while it is inferior in the non-closure of the orbits 

 posteriorly. It may then be regarded as a parallel line. It has but two 

 generic types, while the Oreodontinse present us with seven. So far as yet 

 known, the Agriochcerinse did not continue as long as the Oreodontinfe, 

 as will be shown in tabular form below. 



In the progressive modifications of the Oreodontinse series, the first step 

 was the inflation of the otic bulla (genus Eucrotaphus). This was suc- 

 ceeded by the coossification of the premaxillary bones (genus Mery- 

 cochcerus). These changes were accompanied by a regular increase in 

 dimensions. The species of Merycochoerus are all of the largest size, and 

 there are no small ones. The smallest species of Eucrotaphus are equal to 

 the largest ones of Oreodon. The fourth genus Merychyus, while it loses 

 none of the points already gained, shows a deficiency in its facial walls 

 where vacuities appear. There is the greatest range of size here : with one 

 species (M. major), as large as any of the Merycochceri, we have another 

 as large as the usual Eucrotaphi (M. sygomaticus), and several one degree 

 smaller, or as large as the largest Oreodons. In the next genus the facial 

 vacuities have attained to an enormous size. The premolar teeth become 

 smaller, and the weakness of the narrow symphysis of the lower jaw is 

 made up for by its coossification. The size is reduced from equal to the 

 smallest Merychyi, to that of the smallest Oreodons (genus Leptau- 

 chenia). In the next stage (genus Cyclopidius) the superior incisors dis- 

 appear. Finally, the lower jaw is so reduced in front that it loses both 

 incisors and premolars, in spite of its symphyseal coossification (genus 

 Pithecistes). 



The species may be thus arranged in accordance with their distribution 

 in time. 



White River Epoch. Oreodon gracilis ; 0. affinis ; O. culbertsoni. Eu- 

 crotaphus jacksoni ; E. major. Agriochoerus antiquus ; A. major ; A. 

 latifrons. 



John Day Epoch. Eucrotaphus jacksoni ; E. major. Merycochoerus 

 superbus ; M. leidyi ; M. chelydra, sp. no v. ; M. macrostegus, sp. nov 



