CEPHALOMYS ARCIDENS 



189 



Fig. 118. Right upper premolar, molar 

 series x 4/1. 



Cephalomys arcidens Ameghino 



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



This, the type species, is by far the commonest of the 

 rodents, in fact of all the species in the Deseado, and we 

 found forty-seven specimens 

 on the Chico del Chubut 

 River, west of Puerto Visser. 

 In the species there is con- 

 siderable variation in size for 

 a rodent, but as there are intermediate specimens all the 

 way between the extremes, and as the variation is mostly 



in the size of the fourth 

 premolar, it does not 

 seem proper to separ- 

 ate the material into 

 more than one species. 

 In general, the form 

 has relatively plump 



teeth, relatively heav- 

 ier anct~ thicker than 

 in the other species. 

 Usually the fourth 

 premolar is but little 

 larger than the molars, 



Fig. 119. Left lower premolar, molar series; A, decodu- l-tnf- iti fliic r-Viot-a/^foi- 

 ous premolar 4; B, a little worn mola i ; C, series about l JL 1U l Cl ' 



^^^^^i^^^^tS&l^^. there is considerable 



variation. The fol- 

 lowing measurements give the range of size on the upper 

 jaws: 



Upper premolar 4 to m. 3 

 Upper premolar 4, length 

 Each molar, length 

 Each molar, width 



