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 

 Fig. 127. Right jower / prcmoiar 4 -moiar anterior 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 I, 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. Argcn., 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 



