164 O. O. Marsh — Restoration of Brontops robustus. 



The skeleton represented in this restoration is by far the most 

 complete of any of the group yet discovered. It was found by 

 the author in Dakota, in 1874, and portions of it have been ex- 

 humed at different times since, some of the feet bones having 

 been recovered during the past year. It is a typical example 

 of the family, and shows well the characteristic features of the 

 genus and species which it represents. 



The most striking feature of the restoration here given, aside 

 from the great size of the animal, is the skull. This is sur- 

 mounted in front by a pair of massive prominences, or horn- 

 cores, which are situated mainly on the frontal bones. The 

 nasals contribute somewhat to their base, in front, and the max- 

 illaries support the outer face. These elevations, or horn-cores, 

 vary much in size and shape in the different genera and species. 

 They are always very small in the females. 



The general form of the skull and lower jaw is well shown in 

 the figure. The prominent occipital crest, the widely-expanded 

 zygomatic arches, and the projecting angle of the lower 

 jaw, are all characteristic features. In general shape, the skull 

 resembles that of Brontotherium, but may be readily dis- 

 tinguished from it by the dental formula, which is as follows : 



Incisors f ; canines -f- ; premolars f- ; molars f . 



The presence of four premolars in each ramus of the lower 

 jaw is a distinctive feature in this genus. This character, with 

 the single, well-developed lower incisor, marks both the known 

 species. 



The number of teeth varies in the different genera. The 

 form of the teeth, especially in the molar series, is more like 

 that in Chalicotherium and Diplacodon than in any other 

 known forms. The teeth in the allied genus Brontotherium 

 have already been figured and described by the author. 



The vertebrae are somewhat similar to those of the existing 

 rhinoceros. In the present genus, Brontops, the neural spines 

 of the dorsal vertebras are elevated and massive. There are 

 four sacral vertebrae in this genus, and in the known species 

 the tail is short and slender, as in the individual here described. 



The ribs are* strong and massive. The sternal bones are 

 compressed transversely. The exact form of the first one is 

 not known with certainty, and is here restored from the rhi- 

 noceros. This is the only important point left undetermined 

 in the restoration. 



The fore limbs are especially robust. The humerus has its 

 tuberosities and ridges very strongly developed, and the radius 

 and ulna have their axes nearly parallel. There are four well- 

 developed digits in the manus, the first being entirely wanting. 



