190 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. 
SPECIMEN SPECIMEN AMEGHINO’S 
3089 3058 TYPE 
A SMALL A LARGE 
INDIVIDUAL INDIVIDUAL 
Lower premolar 4 to m. 3 14 mm. 15 mm. 14.5 mm. 
Lower incisor 1 to pm. 4 7+) gins 9 mm. 7.8 mm. 
Height of mandible under pm. 4 7) 9 esrimis 8 mm. 7) mms 
Length of deciduous pm. 4 5.5 mm. 
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 
Ls the molars. The species was 
==" \ not nearly as abundant as C. 
ini, Ho. Richt paldlate dhowihk ore. ET CIdenS, | Occurring but | Srap am 
molar, molar series; external furrows 
appear as pits on molars 2 and 3, x 4/1. times in our collection. 
eae 
Fig. 121. Left mandible, external side, x 4/1. 
