SCOTT: TOXODONTA OF THE SANTA CRUZ BEDS. 
I 2 I 
has been moderately abraded, the two portions of the valley are converted 
into two separate enamel lakes, of which the anterior one is the larger and 
deeper, and the posterior one speedily disappears altogether. In the fully 
adult animal the anterior lake is also worn away, leaving a smooth surface 
of dentine partially enclosed in a wall of enamel. 
The second premolar is considerably larger than p 1 and is inserted by 
two roots ; it has a prominent spur from the external wall, which divides 
the valley into two parts, and a second and much smaller spur projects 
into the valley from the posterior part of the enclosing ridge. Behind 
this ridge is a second valley, which is smaller and much shallower than 
the principal one and has several small, circular enamel-pits in its floor, 
and at the antero-internal angle of the crown is an enamel-lined pocket. 
The third premolar resembles the second, but is somewhat larger, and the 
spur which projects forward from the posterior ridge is better developed. 
After a short period of wear, in both p- and p®, the principal valley becomes 
a narrow, antero-posteriorly directed, enamel-lined slit, and the posterior, 
shallower valley is obliterated, but the pits remain for a time as four or 
five very small enamel-lakes, and the antero-internal pocket persists as a 
lake. The minute lakes of the posterior border soon disappear, but the 
central valley and the anterior lake remain until the animal may be 
called old. 
The fourth premolar, which is the last of the permanent teeth to come 
into use, is of a pattern similar to that of p- and p®, but slightly more 
complicated ; the crista or spur from the external wall is more prominent, 
and that from the posterior crest is divided into three or more ridges, 
while the posterior valley is divided into two well-defined parts, each with 
two or more pits in its floor; the antero-internal pocket is smaller than 
in p®. In quite an advanced state of wear, p- differs from the preceding 
premolars and has a certain resemblance to a worn molar in the retention 
of the crista and the consequent Y-shaped valley. There is, however, no 
real difference of structure and the dissimilar appearance is largely due to 
the very late eruption of p-, so that, at a given stage, it has suffered less 
abrasion than the other premolars. 
The first of the premolar series has an irregularly oval grinding sur- 
face, while in the others this surface is approximately square, but, as the 
crowns contract toward the base, these teeth become smaller through abra- 
sion, as age advances. The external wall of p--p- shows, more or less 
