Hatcher : Little Known Fossil VKRii-.KkAiis. 143 



Length of luiiiuius from proximal to distal articulur sur- 

 faces 290 mm. 



Greatest breadth of distal eiul of humerus 87 mm. 



Greatest length of radius 284 mm. 



Greatest breadth of distal end of radius 73 mm. 



Greatest breadth of proximal end of radius 63 mm. 



Length of ulna 345 mm. 



Distance from anterior border of coronoid jirocess to top of 



olecranon loi mm. 



Depth of ulna opposite coronoid jjrocess 91 mm. 



Length of metacarpal II in mm. 



" " " III ■ 128 mm. 



" " " IV loi mm. 



" " " V 78 mm. 



Combined length of phalanges of digit II 61 mm. 



" an << " " III 61 mm. 



" «' '' << <( li jV 5j n^,^^ 



V 39 mm. 



H ( t 



The Taxonomy of Trigonias osbornl 

 From the above description it will be seen that Trigonias osborfii 

 is the most primitive and least specialized member of the Rhinocero- 

 tidae at present known. Its generalized nature is shown alike by the 

 tetradactyl manus, the unreduced number of superior teeth, and the 

 simple structure of the superior premolars. It falls naturally into 

 Series I of the American Oligocene Aceratheres as defined by Osborn, 

 and from its generalized nature, as well as geological horizon, (base of 

 the Titanotherium Beds), it should be considered as the most primitive 

 form known to that series. It appears to stand directly ancestral to 

 Leptaceratherium trigonodum of Osborn," from which it differs es- 

 sentially in the unreduced superior dentition and the simpler structure of 

 the superior premolars, more especially of P. '--. When the foot structure 

 of the other Aceratheres of this series is definitely known it will prob- 

 ably be found that the manus is tetradactyl in both Leptaceratherium 

 trigonodum and Acerathcriiun mite, while that of ^. platycephalum mdiy 

 perhaps be found to have reached the tridactyl stage. The following 

 diagram is introduced to show at a glance the distinctive characters of 

 the two series of American Oligocene Rhinoceroses proposed by Os- 

 born and to indicate at the same time the geological horizon of the 

 different genera and species contained in each. It is a modified com- 

 bination of the tables introduced by Osborn on pages 126 and 129 of 

 his memoir on e.xtinct Rhinoceroses referred to above. 



" See Mem. of the Am. Mus. of Nat. Hist., Vol. I, Part III, p. 126. 



