CEPHALOMYS 187 
clearly. All the premolars and molars are rooted, though 
the crown is incipiently hypsodont, as much so as in any 
rodent of this period. The incisors are moderately large 
with the anterior face slightly convex, and the antero- 
posterior diameter comparing with the transverse diameter 
as 3 does to 2. The interval between the incisor and 
premolar 4 is moderate, indicating a short snout. 
Each lower molar consists of two transverse laminae 
separated from each other by an internal and an external 
infolding, both of which approach the median line but do 
not meet, a narrow, longitudinal bar separating the folds 
and connecting the anterior and posterior laminae. On 
the inner side, the posterior lamina has a furrow extending 
to the middle of the tooth, but only sinking into the crown 
about a fourth of its height, so that, with wear, it appears 
first as a bay, later as a pit, and finally disappears. In 
general it will be found only on molar 3, and may be 
wanting there on old individuals. On an unworn tooth, 
there occurs, on the inner side of the anterior lamina, 
a rudimentary pit corresponding to the one on the posterior 
lamina, but of much less depth, so that it is only occasion- 
ally seen, and that only on a very slightly worn tooth. 
The premolar differs from the foregoing in having a 
small median column on the anterior face of the anterior 
lamina. 
In three cases we found the deciduous fourth premolar 
(see fig. 119A), a complicated tooth, consisting primarily 
of three laminae in which furrows have developed until 
there are four folds or furrows on the internal side, sepa- 
rating five crests; while on the external side there are three 
furrows and four crests. Ameghino’s figure of this tooth 
in C. prosus has four laminae running clear across the tooth. 
I think the difference is due to his having an unworn decid- 
uous premolar whereas mine are all worn considerably. 
At first glance, the upper teeth appear strikingly differ- 
ent, resembling those of Perimys to which genus they are 
