SINCLAIR I TYPOTHERIA OF THE SANTA CRUZ BEDS. 
17 
the posterior is the smaller. Internally, there is a deep groove opposite 
the external groove just mentioned and another broader but shorter furrow 
anterior to it, which disappears with wear, while the first persists, even in 
well-worn teeth, and is overshadowed anteriorly by a prominent ridge. 
The crown-pattern of the unworn premolars is well shown in No. 9559, 
American Museum, (PI. V, figs. 15, 150) the right ramus of a mandible, 
in which the second premolar is just erupting. On the external side, the 
otherwise convex crown is broadly grooved posteriorly, while internally it 
is rendered concave by the two grooves, already mentioned, of which the 
anterior in the unworn tooth is the broader, deeper, and more conspicuous. 
Consequent upon these grooves three serrations are developed on the 
sharp cutting edge of the crown, of which the second is the most promi- 
nent. Like the premolars, the molars are also bilobate, but the posterior 
lobe is larger than the anterior and in the third molar greatly exceeds the 
latter in size, with a faint trace of a broad external groove and a deeper, 
more persistent groove internally. The worn surfaces of the anterior and 
posterior lobes in the first and second molars are triangular in outline 
with elevated corners, inclosing a basin-shaped depression. This applies 
also to the anterior half of the third molar, but the posterior half is irregu- 
larly elliptical in outline and the depression on the triturating surface has 
its margins interrupted by the shallow grooves described above. Internally, 
the molars exhibit the same persistent groove, with anterior bounding 
ridge, as in the premolars. The ectoloph on either side of this groove is 
slightly concave. Both molars and premolars are hypsodont and cement- 
covered. In the immature individual figured on Plate V, figures 13, 14, 
the last molar is just appearing above the alveolar border and the tooth 
preceding it is but little worn. Each molar is composed of a pair of cres- 
centic lobes concave internally. The anterior crescent is breached on the 
inner side by a broad groove corresponding to the shallow anterior groove 
already mentioned in describing the unworn premolars. The cutting edge 
of this crescent is sharp, forming a high external cusp. The anterior horn 
is recurved, terminating in a small blunt tubercle. The posterior horn is 
produced beyond the anterior extremity of the posterior crescent, giving 
rise to the persistent internal ridge and groove observable even in worn 
molars. The posterior lobe incloses a depression bounded by sharp edges, 
formed by the postero-external crescent and a long straight ridge (text fig. 
6, A, posterior pillar) trending posteriorly from the anterior horn of the 
