£>'0 The Badlland Formations of the Black Hills Region 



are especially characterized as being without horns, hence the 

 old name Acerethere. The Miocene forms have generally, but 

 not always, a rudimentary or fairly well developed pair of horns 

 paced transversely across the anterior part of the head, hence 

 the name Dicerathere. Present day rhinoceroses, it should be 

 remembered, have either no horn or one or two horns, but the 

 arrangement when horns are present is always medial, never 

 transverse. It is of interest to note also that while all living 

 rhinoceroses have feet that are functionally tridactyl, some of 

 the ancestral true rhinoceroses, at least so far as concerns the 

 front feet, were functionally tetradactyl. This is known to be 

 true of Trigonias osborni, and is suspected of others. This 

 lessening of the number of functional toes corresponds to similar 

 alterations in other animals and indicates progressive change. 

 Indeed, the rhinoceroses show in many ways gradual trans- 

 formations, particularly with reference to the feet, the teeth, 

 and the development of horn cores. Osborn states that Caeno- 

 pus copei, by a beautfiul series of transitions, passes into' Caeno- 

 pus occid entalis, and this in turn, by steady evolution through 

 stages which might well be considered of specific value into 

 Caenopus tridactylus. Likewise Trigonias osborni, the most 

 primitive and least specialized true rhinoceros known, appears 

 to stand directly , ancestral to Leptaceratherinm trigonodum. 

 Again the Aceratheres in their later history developed nasal 

 rugosities, and there is reason to believe that from their stock 

 the Diceratheres developed. 



Among the Aceratheres Caenopus mitis was the smallest, 

 its height at the shoulders being approximately twenty-eight 

 inches. Among the Diceratheres Diceratherium schiffi was the 

 smallest. It was also most specialized. The largest of the Acer- 

 atheres, in fact the largest of all the true rhinoceroses, seems to 

 have been Caenopus platycephalus. It considerably surpassed 

 the present day Sumatran rhinoceros. Among the others Caeno- 

 pus copei was about the size of the American tapir and Caenopus 

 tridactylus, reproduced on Plate 28, was nearly as large as the 

 Sumatran rhinoceros. The latter specimen measures seven feet, 

 nine inches in length, and four feet high to top of the rump. 



The following list gives the species of the true rhinoceroses 

 as well as the Hyracodonts and Amynodonts found in the 

 badland formations of the Black Hills region : 

 Lower Oligocene. 



Hyracodontidae. 



Poor material, not named specifically. 



