4 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
2. Dentition. — Beginning with the normal incisor formula in Protypo- 
therium, the Santa Cruz typotheres show a well-marked tendency toward 
an increase in size of the median pair at the expense of the lateral incisors, 
the canine and the anterior premolar, until the extreme stage of reduction 
in Pachyrukhos is attained. The teeth approaching elimination are re- 
duced to simple cylinders. It is not to be understood, however, that 
Fig. i. 
Skulls of Santa Cruz Typotheria, three-fourths the natural size. A, Protypotherium australe (No. 
9565, American Museum of Natural History collection) ; B, Interatherium robustum (No. 9263 
American Museum of Natural History collection) ; C. Pachyrukhos moyani (reconstructed from 
several specimens) ; D, Hegetotherium mirabile (No. 15,542 Princeton University). 
Protypotherium , Interatherium , Hegetotherium and Pachyrukhos constitute 
a phyletic series because they represent successive stages in this process 
of dental reduction associated with the hypertrophy of the median incisors. 
As already indicated in the generic key, two divergent lines are repre- 
sented. A rather curious feature of the lower incisors in Protypotherium 
