CEPHALOMYS ARCIDENS 189 
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 sizeof 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, 
ie pe Leth lowempr molar molar suey decodu  itein, this, character, 
ous premolar 4; B, a little worn mola 1; C, series about 
half worn down; a.l., anterior lamins; /.l., posterior Da 4 Sea led s 
lamina; 1.f., internal fold; e.f., external fold; p.g., fur- there Is ¢ onsiderable 
row in posterior lamina; x 4/T1. variation. ah h e fol 4 
lowing measurements give the range of size on the upper 
jaws: 
Fig. 118. Right upper premolar, molar 
series x 4/1. 
SPECIMEN SPECIMEN AMEGHINO'S 
3109 3099 TYPE 
A SMALL A LARGE 
[INDIVIDUAL INDIVIDUAL 
Upper premolar 4 to m. 3 12.5 mm, 13.5 mm. 13.5 mm. 
Upper premolar 4, length 3.5 mm. 4.5 mm. 
Each molar, length 3. mm. 3) mim. 
Each molar, width 2.75mm. 2) ain. 
