348 Annals of the Carnegie Museum. 



probable that the subtropical conditions which prevailed over this 

 region in Eocene and early Oligocene times became so altered toward 

 the middle of the latter period that the palms and other subtropical 

 plants known to have lived in those times were replaced by the short, 

 hardy grasses and low straggling bushes so characteristic of semi-arid 

 plains wherever found. Perhaps some such changes as those just sug- 

 gested brought about the extermination of the Titanotheres. How- 

 ever this may be, it is certain that these animals lived in great abun- 

 dance over that region now occupied by our Western plains during 

 the deposition of the Titanotherium beds which lie at the base of the 

 White River series, and their remains now ap])ear as the most abun- 

 dant fossils of these deposits, indicating that their extermination was 

 quite sudden rather than gradual, though the latter method would 

 seem to have been the one most usually followed by nature in the 

 process of the elimination of any particular group of animals from the 

 fauna of a region, the individuals of such group gradually becoming 

 less frequent and finally disappearing altogether. For while remains 

 of these animals are exceedingly abundant throughout the strata per- 

 taining to the Titanotherium series, not a fragment of Titanotherium 

 has ever been found in the Oreodon beds which immediately overlie 

 the former series and have generally been considered as the result of a 

 continuous deposition going on uninterruptedly from the base of the 

 one to the top of the other, though the present writer has maintained 

 that there are important stratigraphical and faunal evidences in favor 

 of a considerable hiatus between these two series. This would 

 obviate the necessity of assuming the sudden disappearance of the 

 Titanotheres and permit of the possibility of their gradual extermina- 

 tion, after the manner which has been so logically and graphically 

 shown by Darwin to have l)een the case with other groups of extinct 

 animals. 



While the remains of these animals are (juite common they consist 

 for the most part of isolated skulls and bones, with an occasional limb 

 and foot. Only rarely have even fairly complete skeletons been found, 

 and hitherto there has existed in our museums but one mounted skele- 

 ton, that of the American Museum of Natural History in New York. 



While collecting vertebrate fossils from the Tertiary deposits of 

 Sioux Co., Nebraska, in August, 1900, the present writer discovered 

 near the base of the Titanotherium beds on Warbonnet Creek, some 

 three miles north of the Brewster and Emmons Ranch, an unusually 



