194 THE DESEADO FORMATION OF PATAGONIA 
Litodontomys chubutensis sp. nov. 
The type is number 3086 of the Amherst collection, 
from the Deseado beds on the Chico del Chubut River, west 
of Puerto Visser, and consists of 
the lower premolar-molar series. 
Premolar 4 is elongated, the 
Ee. 27, SRN oes renee ees Han eerlOt lamina being consider- 
ably longer than it is wide, 
whereas the laminae of the other teeth are wider than they 
are long. 
The following measurements with the figure give the 
specific details. 
Lower dentition, premolar 4, to molar 3 10.5 mm, 
Lower dentition, premolar 4, length 3.5 mm 
Lower dentition, molar 1, length 2. mm 
Lower dentition, molar 2, length 2.5 mm 
Lower dentition, molar 3, length 2.5 mm 
Lower dentition, width of molars 2. mm 
ERETHIZONTIDAE 
Asteromys Ameghino 
Asteromys Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 495. 
The genus contains small forms, with brachydont teeth. 
The upper molars consist of two laminae, separated by an 
internal and an external fold, and each lamina having, 
on the internal half, a deep furrow, which is but little 
shallower than the median fold; so that the outer side 
shows three furrows, folds, or pits, more or less completely 
separating four lobes; while on the inner side of the tooth 
there are but two lobes. On the lower teeth the external 
median fold is deep, while the internal median fold is 
shallower, usually appearing as a pit. Both the anterior 
and posterior laminae are subdivided by wide internal 
furrows which extend to the median line. 
These characters associate the genus with Acaremys of 
the Santa Cruz, from which genus it is not easy to separate 
