188 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. 
UPPER PM. 4 LOWER PM. 4 
TO M. 3 TO M. 3 
C. arcidens 13-14 mm. 14-15 mm, 
C. plexus 9.5 mm. 10.5 mm. 
C,. prosus 8.5 mm. 9.5 mm. 
