GLIRES OF THE SANTA CRUZ BEDS. 399 



It is exceedingly difficult, if not quite impracticable, to determine which 

 of these similarities are due to genetic relationship and which of them to 

 convergent development. On the whole, it seems probable that in Neor- 

 eomys we have the somewhat modified remnant of a more ancient group, 

 which was the common ancestor of the three families indicated. 



NEOREOMYS AUSTRALIS Ameghino. 



(Plates LXIV, figs. 6-6" ; LXV, i-ii ; LXX.) 



Neoreomys australis Amegh.; Enumeracion sistemat, etc., 1887, p. 11. 



Neoreomys ind^ms^^s Amegh. ; Ibid. 



Pseudoneoreomys leptorhynchus Amegh. ; Rev. Argent, de Hist. Nat. ; T. I, 



1891, p. 300. 

 Pseudoneoreomys mesorhynchus Amegh.; Ibid., p. 301. 



This is by far the most abundant and best known species of the genus 

 and from it most of the preceding generic description has been taken. In 

 size, the species is very variable, some individuals much surpassing others 

 in this respect. Much of this variation is, however, due to differences of 

 age, as growth evidently continued till a comparatively late period of 

 life, and every stage of transition between the extremes may be observed. 

 The teeth, on the other hand, are quite constant, except as their form and 

 size are altered by abrasion. 



, The incisors have slightly, but distinctly, convex faces and p T has no 

 groove upon the outer side of the anterior prism. 



In the following table, Nos. 15,222 and 15,396 are young individuals, 

 15,572 and 15,365 adult, while 15,266 was an old animal with much worn 

 teeth. 



MEASUREMENTS. 



No. No. No. No. No. 



