196 THE DESEADO FORMATION OF PATAGONIA 
Eosteiromys Ameghino 
Eosteiromys Amegh., 1902, Bol. Acad. Nac. Cienc., Cordoba, t. 17, p. 10. 
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, 7. ¢., it 
is divided into an anterior and posterior lamina by a deep 
external median fold and by a shorter oblique internal me- 
dian fold. The anterior lamina is subdivided by two ex- 
ternal furrows, a lesser anterior and a larger posterior; 
while the posterior lamna 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 
? 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 
Big. 13) ees “molar 2, length. 4 mm., width 5 mim, 
