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. 1 to m. 3 55 mm 
Upper dentition, length pm. I to m. 3 31) am 
Upper dentition, incisor 1, width 6} mm. 
Upper dentition, molar 1, length 6 mm. 
Upper dentition, molar, width 4; mm. 
Mandible, greatest length ; 82 mm. 
Lower dentition, length inc. 1 to m. 3 53 mm 
Lower dentition, length pm. I to m. 3 32 mm. 
Lower dentition, molar 1, length 6; mm. 
Lower dentition, molar, width 2) anim 
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 go mm. 
Astragulus, length 14 mm. 
Astragulus, width It mm. 
Calcaneum, length 252 mm. 
Metatarsus ITI, length 32 mm. 
First phalanx of digit III, length 12 | mim. 
Ungual palanx of digit III, length g 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. 
