SINCLAIR— THE SANTA CRUZ TYPOTHERIA. 



71 



closely resembles that of the rabbit. From the numerous structural 

 similarities between Pachyrukhos and Hcgetherium it may be in- 

 ferred that the latter was also saltatorial. Its broad, shallow as- 

 tragalar trochlea is in contrast with the narrow, more deeply incised 

 trochlea of the cursorial Protypotherimn and Interatherium. Both 

 of these genera have the fore and hind limbs of approximately equal 

 length. The terminal phalanges in the Santa Cruz typotheres are 

 hoof-like and in Hcgctotherium have prominent median clefts. 



Relationships of the Santa Cruz Typotheria. 

 I. With the Toxodonta. — In the evolution of the teeth and feet, 

 the Santa Cruz Typotheria are less advanced than their contempor- 

 aries, the Nesodons. The feet of Kcsodon (Figs. 6, B, y, B) are 



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Fig. 6. A. Left fore foot of Protypotherium australe Ameghino, three 

 fourths the natural size. (No. 9149 American Museum of Natural History- 

 collection.) B. Left fore foot of Nesodon imbricattis Owen, about one fifth 

 the natural size. (No. 15460 Princeton University collection.) 



tridactyle with the axis passing through the third digit. The manus 

 has originally been tetradactyl like that of Protypotherium (Fig. 

 6, A) but has lost almost all trace of the fifth digit, a mere vestige, 

 not shown in the figure, remaining. The other bones of the wrist 

 and foot have not suffered any displacement as a result of this loss 

 but interlock in the same way as in Protypotherium. The hind foot 



