| Upper pm. 4 to m. 3 | Lower pm. 4 to m. 3 | |
|---|---|---|
| C. arcidens | 13-14 mm. | 14-15 mm. |
| C. plexus | 9.5 mm. | 10.5 mm. |
| C. prosus | 8.5 mm. | 9.5 mm. |
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 considerable 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 separate the material into more than one species.
Fig. 118. Right upper premolar,
molar series × 4/1.
Fig. 119. Left lower premolar, molar series;
A, deciduous premolar 4;
B, a little worn molar 1;
C, series about half worn down;
a.l, anterior lamina;
p.l. posterior lamina; i.f., internal fold;
e.f., external fold; p.g., furrow in
posterior lamina; × 4/1.
In general, the form has relatively plump teeth, relatively heavier 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 following measurements give the range of size on the upper jaws: