CEPHALOMYS ARCIDENS 



189 



Right upper premolar, molar 

 series x 4/1. 



Cephalomys arcidens Ameghino 



C. arcidens Amegh., 1897, Bol. Fnst. 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 and thicker than 

 in the other species. 

 Usually the fourth 

 premolar is but little 

 larger than the molars, 

 but, in this character, 

 there is considerable 

 variation. The fol- 

 lowing measurements give the range of size on the upper 

 jaws: 



Fig. 1 10. Left lower premolar, molar series; .4 , decodu- 

 ous premolar 4; B, a little worn mola 1; C, series about 

 half worn down; a.l., anterior lamins; p.l., posterior 

 lamina; «'./., internal fold; «./., external fold; p.g., fur- 

 row in posterior lamina; x 4, 1. 



