Premolar 4 is elongated, the anterior lamina being considerably longer than it is wide, whereas the laminae of the other teeth are wider than they are long.
The following measurements with the figure give the specific details.
| Measurements Specimen 3063 | ||
|---|---|---|
| Lower dentition, | premolar 4, to molar 3 | 10.5 mm. |
| Lower dentition, | premolar 4, length | 3.5 mm. |
| Lower dentition, | molar 1, length | 2. mm. |
| Lower dentition, | molar 2, length | 2.5 mm. |
| Lower dentition, | molar 3, length | 2.5 mm. |
| Lower dentition, | width of molars | 2. mm. |
ERETHIZONTIDAE
Asteromys Ameghino
Asteromys Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 495.
The genus contains small forms, with brachydont teeth. The upper molars consist of two laminae, separated by an internal and an external fold, and each lamina having, on the internal half, a deep furrow, which is but little shallower than the median fold; so that the outer side shows three furrows, folds, or pits, more or less completely separating four lobes; while on the inner side of the tooth there are but two lobes. On the lower teeth the external median fold is deep, while the internal median fold is shallower, usually appearing as a pit. Both the anterior and posterior laminae are subdivided by wide internal furrows which extend to the median line.
These characters associate the genus with Acaremys of the Santa Cruz, from which genus it is not easy to separate Asteromys; but as we know only the teeth from the Deseado beds, it is probable that, when the skull is found, larger differences will be recognized. Asteromys appears to be the direct ancestor to Acaremys.
The species are all tiny, the following three being distinguished by Ameghino.
| Length of Lower Molar Series | |
|---|---|
| A. punctus (Bol. Inst. Geog. Argen., 1897, t. 18, p. 495) | 12 mm. |
| A. annectens (Bol. Acad. Nac. Cienc. Cordoba, 1902, t. 17, p. 37) | 11 mm. |
| A. prospicuus (Bol. Inst. Geog. Argen., 1897, t. 18, p. 495) each molar | 1.6 to 1.8 mm. |