IQO 



THE DESEADO FORMATION OF PATAGONIA 



The lower jaw is low, heavy and rather short, the 

 posterior part of the ramus being very thin, while the 

 portion carrying the teeth is thick and heavy. A strong 

 ridge extends along the inner side from just behind molar 3 

 to the base of the symphysis. As in the upper dentition, 

 there are smaller and larger forms. 



Cephalomys plexus Ameghino 



C. plexus Amegh., 1897, Bol Inst. Geog. Argen., t. 18, p. 494. 



In general, this species is similar to the foregoing, but 



is smaller in size and slenderer 

 in proportions. Both the upper 

 and lower fourth premolar tend 

 to be considerably larger than 

 the molars. The species was 

 not nearly as abundant as C. 

 , arcidens, occurring but sixteen 



molar, molar series; external furrows ,•_ • „..„ „^M,^t-l^ n 



appear as pits on molars 2 and 3, x 4/1. tlltlCS in OUr Collection. 



Fig. 121. Left mandible, external side, x 4/1. 



