F. B. Loomis—Rhinocerotidce of the Lower Miocene. 53 



During Oligocene times the country west of the Great Lakes 

 and either side of the Canadian line seems to have been teem- 

 ing with Aceratheres, abundant in numbers and varying in 

 characteristics. At the end of the Oligocene all but a remnant 

 of this rich fanna disappeared, its descendants in Europe still 

 flourishing while but a handful still held on in America, as will 

 be shown later. Why the disappearance is unknown. Directly 

 succeeding these Aceratheres, it now appears that the Dicera- 

 theres flourished, apparently as rich in numbers and in species. 

 While longer limbed and somewhat shorter headed, the den- 

 tition forbids any thought that these were open country crea- 

 tures. There are no nibbling teeth and the backs of the canines 

 are worn as when branches are stripped of leaves by drawing 

 them through the mouth. The grinders are also those of a 

 brouser. Spreading westward and northward, these Dicera- 

 theres crossed the Berings isthmus and reached Germany and 

 France, there to become in a short, time extinct. In America 

 they multiplied in Lower Miocene times, and in the Harrison 

 period no less than five* species were ranging over Nebraska 

 and Wyoming. In their turn the Diceratheres as mysteriously 

 wane and die out, the last one known being only indicated by 

 a single tooth from the Upper Miocene of Oregon. 



Throughout the genus, the size and shape of the nasal horns 

 vary in any species with the age and sex of the individual. 

 Variation is also characteristic in the weight and stockiness of 

 the skull as a whole. In all features there is that tendency to 

 fluctuation which is found in a young and developing group, 

 the different species representing apparently points in evolu- 

 tionary lines. Some of the species can be gathered into sub- 

 groups on common features which represent common descent, 

 but there are still many gaps to be filled before a perfect case 

 of adaptive radiation will be illustrated. The characters which 

 have proven most satisfactory for establishing species are, after 

 size and contour of the skull has been considered in a general 

 way, the pattern of the premolar and molar teeth. 



In the following descriptions the Osborn nomenclature has 

 been used, a key to which is given in fig. 1, p. 51. The figures 

 are all one-half natural size except fig. 10. Further measure- 

 ments are given in the table at the end of the descriptions of 

 the genus Diceratherium. 



Diceratherhim armatum Marsh. 

 This Journal, vol. ix. p. 242, 1875. 



The type is Xo. 10,003 in the Yale Museum, a complete skull 

 somewhat crushed dorso-ventrally, from " near John Day River 

 in Eastern Oregon. " 



* Probably as many more will turn up within a few years, judging from 

 the variation of toe bones and other of the less characteristic features. 



