192 THE DESEADO FORMATION OF PATAGONIA 
the sixteen which we found would hardly represent the 
real proportion of the species in the fauna. 
The jaws are not only small, but also slender and deli- 
cately built, with the premolar about the same size or 
slightly larger than the molars. The drawings represent 
the proportions accurately so I will give but a few measure- 
ments. 
SPECIMEN AMEGHINO’S 
3009 TYPE 
Upper premolar 4 to molar 3 8.5 mm, 
Lower premolar 4 to molar 3 9.5 mm, 
Scotamys gen. nov. 
A lower jaw, with premolar 4 and molars 1 and 2, from 
the Deseado beds on the Chico del Chubut River, west of 
Puerto Visser, indicates a genus of hystricomorph rodents 
not previously reported. The lower molars suggest those 
of Perimys, but premolar 4 is similar to that of Scotaeumys 
from the Santa Cruz. Scotamys differs, however, from 
Scotaeumys, in that its molars do not have the third lobe 
found in the Santa Cruz genus. I have, therefore, made 
a new genus which appears to be ancestral to Scotaeumys. 
Scotamys antiquus sp. nov. 
This, the type species of the above genus, is based on 
specimen 3063, a lower jaw with the incisor, premolar 4 
and the first two molars. The incisor 
is fairly large and heavy, the anterior 
face slightly convex, and the anterio- 
posterior diameter greater than the 
transverse diameter. Premolar 4 hasa 
deep external fold, dividing the crown into an anterior 
and posterior lamina, the former being then subdivided 
by another external fold, making the tooth three-lobed. 
Just internal to the median fold is a tiny pit, apparently 
the last vestige of an internal fold. Each molar consists 
Fig. 125. Lower premolar 4 
—molar 2, x 4/I. 
