i9o8.] SINCLAIR— THE SANTA CRUZ TYPOTHERIA. 75 



degree of specialization in foot structure displayed by Typotherium 

 may require some modification in the light of fuller knowledge. 



3. With the Rodents. — In many features of skull and skeleton 

 the Typotheria resemble the rodents. This is most apparent in 

 Pachyrnkhos, which seems to have been a saltatorial animal, but in 

 none of the Typotheria are the following characters peculiar to 

 rodents developed : 



A. Persistently growing, chisel-shaped incisors (I. % of the per- 

 manent series, Weber).*' L % of the permanent series is enlarged 

 in some of the Typotheria and may grow persistently but is modified 

 for cropping and not for gnawing. 



B. More or less antero-posterior elongation of the mandibular 

 condyle and corresponding modification of the glenoid fossa to 

 permit backward and forward movement of the lower jaw. In the 

 Typotheria the condyle is approximately circular in outline with the 

 glenoid surface flattened and the movement of the mandible is from 

 side to side. 



C. Frequent outward curvature of the crowns of the upper 

 molars and inward curvature of those of the inferior series in hypso- 

 dont forms. The reverse is true in the Typotheria. 



D. Contact of ascending process of premaxillary with frontal. 

 This process is short and robust in the Typotheria and is widely 

 separated from the frontal by the maxillary. 



E. Elongation of the mandibular angle. The angle is evenly 

 convex in the Typotheria. 



F. The astragalus in rodents is characterized by a broad, short, 

 rather shallow trochlea with the crests sharp and equally developed, 

 distinct neck and flattened head, convex distally; trochlea symmet- 

 rical to the vertical plane ; fibular and internal malleolar facets ver- 

 tical ; body limited posteriorly ; no astragalar foramen. In the Santa 

 Cruz Typotheria the body is deeper than in rodents, the crests may 

 or may not be equally developed and the head is globular without 

 antero-posterior flattening. The symmetry of the trochlea with 

 respect to the vertical plane varies in the different families. In the 

 other characters they resemble rodents. 



" " Die Saugetiere," p. 480, 1904. 



