scott: toxodonta OF THE SANTA CRUZ beds. 
153 
some individuals the two fossae communicate through a large perforation, 
but in others they are completely separated by a thin bony wall. The 
trochlea is wide and low and is divided into an external convexity for the 
head of the radius and a broad, saddle-shaped area for the ulna, which 
is bounded internally by a prominent flange. 
Fig. 23. 
Toxodon burmeisteri : Left humerus, dorsum, 
X T La Plata Museum. 
Fig. 24. 
Nesodon imbricatus : Left humerus, dorsum, 
xh 
In Toxodon the humerus has become enormously massive and is rela- 
tively very short ; the external tuberosity is proportionately lower than in 
Nesodon and is shifted more toward the outer side. The bicipital groove 
is divided into a very broad external and narrower internal portion and 
the former is again subdivided by a low, median bicipital tubercle. The 
deltoid area develops a low, but very massive and rugose, external hook. 
The supinator ridge is prominent and the external epicondyle very large 
and heavy ; the radial portion of the trochlea is high and narrow and 
