E. L. Troxell—Ccenopus, the Ancestral Rhinoceros. 41 



Art. lY.—CcEnopus, the Ancestral Rhinoceros; by 

 Edward L. Troxell. 



[Contributions from the Othniel Charles Marsh Publication Fund, Peabody 

 Museum, Yale University, New Haven, Conn.] 



Introduction. 



Until very recent times there were two great groups of 

 extinct rhinoceroses mentioned in the literature, Acera- 

 therium Kaup and Diceratherium Marsh, and specimens 

 from Lower Oligocene to Middle or Upper Miocene, both 

 in the Old and New World, were classified according to 

 the nasal bones, whether or not they had rugose thicken- 

 ings designed to support horns. Due to the work of 

 Osborn, Scott, Loomis, Cook, and especially of Peterson, 

 it now appears that the two classes are simply the hornless 

 females and the horned males, of a variety of genera. 

 There are, however, two important exceptions to this 

 general rule: (1) the early Oligocene species which did 

 not show the horn rugosities in the males, and (2) those 

 recent animals ("excepting Rhinoceros sondaicus) in 

 which both males and females may have horns. 



Peterson shows that in Diceratherium cooM the horns 

 belonged to the mature males alone; the females and 

 young males were hornless. In the Peabody Museum 

 there are horned and hornless specimens of Diceratherium 

 from the John Day beds of Oregon. The mature animals 

 may be either, but the very young individuals always 

 have smooth nasals. Osborn (1898) has demonstrated 

 that Ccenopus tridactylus also has this sexual distinction, 

 following the discovery by Hatcher (1894) of "D." pro- 

 avitum with horns in the males in a very primitive state. 

 The name of Diceratherium therefore ceases to have its 

 original sense, all inclusive, and is now limited to one 

 phase of the horned rhinoceroses, the type of which is D. 

 armatum Marsh. Other species of ^'diceratheres" may 

 be, and some are, widely separated in their classification, 

 as will be shown later. 



^ For obvious reasons, space is not given to the publication of all refer- 

 ences; the reader is therefore directed ta the memoirs by Osborn (^'The 

 extinct rhinoceroses ' % Mem. Amer. Mus. Nat. Hist,, vol. 1, 75-164, pis. 

 12A-20, 1898) and by Peterson (^'The American diceratheres ", Mem. Car- 

 negie Mus., vol. 7, 399-477, pis. 57-66, 1920), in which detailed descriptions, 

 fine reproductions of all important types, and full bibliographies are pub- 

 lished. 



