142 Annals of the Carnegie Museum, 



of the upper anterior surface of the magnum. This extensive, contin- 

 uous articular surface is excei)tionally regular in outline, and its an- 

 terior edge describes an almost perfect semicircle. Superiorly the unci- 

 form articulates subequally with the lunar and pyramidal. 



The metacarpus. Although tetradactyl in structure, the manus of 

 Tri'i-onias shows a decided tendency in the direction of tridactylism. 

 Metacarpal III is the longer and much the stronger bone of the series, 

 while metacarpals II and IV are subequal. The inevitable suppres- 

 sion to which the fifth digit is destined, is already distinctly indicated 

 both by the inferiority of its metacarpal and by that of the succeeding 

 phalanges. Nevertheless, there is still a very considerable modification 

 necessary before the manus of Trigonias assumes the typically tridactyl 

 structure of the later Rhinoceroses and Aceratheres. Metacarpal III is 

 decidedly asymmetrical, and the axis of the foot is external to its 

 median line, while the superior anterior surface of the magnum is not 

 so completely occupied by the scaphoid as in the later tridactyl forms. 



The phalanges. As in all the Rhinocerotidae, the phalanges are 

 short and rather flat, with rugose upper surfaces. The unguals, espe- 

 cially those of digits II, III, and IV, are broad and very short. 

 There ar^ present the metacarpophalangeal and navicular sesamoids 

 characteristic of Perissodactyls in general. 



The material, upon which the above description of the scapula, 

 fore limb, and manus is based, consists of a complete right manus, 

 limb, and scapula (No. 95) found in position along side of the skulls 

 (Nos. 96 and 98) described above. It appears to pertain to the same 

 individual as No. 96, and associated with it was the opposite fore- 

 limb, pelvis, ribs, vertebrce, hind limbs, and most if not quite all of 

 the skeleton, for the most part disarticulated. All this material, in- 

 cluding the lower jaw described above, was taken from a single bone 

 deposit, near the base of the Titanotherium Beds, not exceeding in 

 area a space twenty feet in length by ten in width, and all from the 

 same horizon. The locality is about three miles north of the old 

 Brewster and Emmons Ranch, on Warbonnet creek, Sioux County, 

 Nebraska. Front and side views of the manus are shown on Plate 

 III, Figs. I and 2, and an external side view of the manus, scapula, 

 and limb in position on Plate IV. 



Measurements. 



Length of scapula 3^7 "i™- 



Greatest breadth of scapula 1 60 mm. 



