| Specimen 3109 a Small Individual | Specimen 3099 a Large Individual | Ameghino’s Type | |
|---|---|---|---|
| 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 mm. | |
| Each molar, width | 2.75 mm. | 3 mm. |
The lower jaw is low, heavy and rather short, the posterior part of the ramus being very thin, while the portion carrying the teeth is thick and heavy. A strong ridge extends along the inner side from just behind molar 3 to the base of the symphysis. As in the upper dentition, there are smaller and larger forms.
| Specimen 3089 a Small Individual | Specimen 3058 a Large Individual | Ameghino’s Type | |
|---|---|---|---|
| Lower premolar 4 to m. 3 | 14 mm. | 15 mm. | 14.5 mm. |
| Lower incisor 1 to pm. 4 | 7 mm. | 9 mm. | 7.8 mm. |
| Height of mandible under pm. 4 | 7 mm. | 8 mm. | 7 mm. |
| Length of deciduous pm. 4 | 5.5 mm. |
Cephalomys plexus Ameghino
C. plexus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 494.
In general, this species is similar to the foregoing, but is smaller in size and slenderer in proportions. Both the upper and lower fourth premolar tend to be considerably larger than the molars. The species was not nearly as abundant as C. arcidens, occurring but sixteen times in our collection.
Fig. 120. Right palate showing premolar,
molar series; external furrows appear as
pits on molars 2 and 3, × 4/1.
Fig. 121. Left mandible, external side, × 4/1.