1 88 THE DESEADO FORMATION OF PATAGONIA 



probably ancestral. Each molar consists of two laminae, 

 separated by a deep internal fold which extends almost 

 to the external margin. On little worn teeth each lamina 

 shows, on the external side, a shallow furrow extending 

 to about the middle of the tooth, but these furrows early 

 become pits and then disappear with further wear, being 

 preserved on not over a fourth of our specimens. The 

 fourth upper premolar consists of two laminae, but in this 

 case, the separating fold is on the external side and extends 

 nearly to the internal margin, so that this tooth appears 

 to be reversed in its position in the jaw. As in the molars, 

 there is, on the external side of either lamina, a furrow, 

 the one in the anterior lamina shallow and seldom seen, 

 that in the posterior lamina deep and present in all but 

 the most worn teeth. 



While the upper and lower molars appear so different 

 they may be readily derived from such a tooth as the 

 lower molar, as both have the two laminae and separating 

 furrows in common. In the upper molars, however, the 

 internal fold is prolonged until the external fold is merely 

 indicated or lacking. On upper premolar 4, on the con- 

 trary, it is the external fold which is prolonged. The fur- 

 rows in the external portions of the laminae of the upper 

 molars correspond to those on the internal portions of 

 the same laminae of the lower teeth, reversed, as is typical 

 of all teeth. 



Ameghino distinguished three species of Cephalomys, 

 which are based primarily on size, the other characters 

 which he gave being inconstant. We found these three 

 and no others. 



C. arcidens 

 C. plexus 

 C. prosus 



