196 THE DESEADO FORMATION OF PATAGONIA 



Eosteiromys Ameghino 



Eostciromys Amegh., 1902, Rol. Acad. Nac. Cienc, Cordoba, t. 17, p. 1 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, i. e., 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 

 F,s nVolar R 2 i8 x 4A ,I,ur molar 2, length 4 mm., width 5 mm. 



