70 THE DESEADO FORMATION OF PATAGONIA 



the second series, all associated, which probably indicates 

 the full number of the toes. The ungual phalanges are 

 proximally narrow and high, then expand toward the tip, 

 developing into marginal expansions. There is but a trace 

 of a cleft in the end of these ungual phalanges. 



MEASUREMENTS 



Skull, greatest length 99 mm. 



Upper dentition, length inc. I to m. 3 55 mm. 



Upper dentition, length pm. I to m. 3 31 mm. 



Upper dentition, incisor I, width 6\ mm. 



Upper dentition, molar I, length 6 mm. 



Upper dentition, molar, width 4? mm. 



Mandible, greatest length 82 mm. 



Lower dentition, length inc. I to m. 3 53 mm. 



Lower dentition, length pm. I to m. 3 32 mm. 



Lower dentition, molar I, length 6| mm. 



Lower dentition, molar, width 3 mm. 



Third metacarpus, greatest length 28 mm. 



Pelvis, length front to back 83 mm. 



Femur, greatest length (computed) 93 mm. 



Femur, diameter of middle of shaft 9 mm. 



Tibia, greatest length 90 mm. 



Astragulus, length 14 mm. 



Astragulus, width II mm. 



Calcaneum, length 25^ mm. 



Metatarsus III, length 32 mm. 



First phalanx of digit III, length 12 mm. 



Ungual palanx of digit III, length 9 mm. 



To make the similarity of Prosotherium with Pachy- 

 rukhos clearer, I have restored Prosotherium, figure 40, 

 from which it will be seen that this genus is also a hopping 

 form with a plantigrade hind foot and a semidigitigrade 

 front foot. In general it compares very closely with Pachy- 

 rukhos, but the limbs are shorter and the grade of speciali- 

 zation is not quite as high. It is, however, very evidently 

 the ancestor of Pachyrukhos. 



