Eosteiromys Ameghino
Eosteiromys Amegh., 1902, Bol. Acad. Nac. Cienc., Cordoba, t. 17, p. 110.
The genus was established by Ameghino for forms similar to Steiromys of the Santa Cruz, but antedating that time. I confess I can see but little difference between Steiromys and Eosteiromys, but the latter is as yet known only from isolated teeth and as in general it would be expected that there should be a generic difference, we may let this genus stand representing rather a prophecy than the facts as yet known.
The upper teeth are brachydont, the crown being on the same general plan as in the foregoing genus, i. e., it is divided into an anterior and posterior lamina by a deep external median fold and by a shorter oblique internal median fold. The anterior lamina is subdivided by two external furrows, a lesser anterior and a larger posterior; while the posterior lamina is subdivided by a single external furrow; so that this tooth has four folds, furrows, or pits on the external side separating five lobes; while on the inner side there is but the one oblique fold separating two lobes.
Eosteiromys medianus Ameghino
Fig. 130. Right
upper molar,
2 × 4/1.
? E. medianus Amegh., 1902, Anal. Mus. Nac. Buenos Aires, ser. 3, t. 2, p. 129.
The genus is based on a single, though entirely characteristic, upper molar. We found just one tooth of the species, also an upper molar. It is described above, under the genus. The measurements are: Upper molar 2, length 4 mm., width 5 mm.