Sinclair: typotheria of the santa cruz beds. 
15 
studied these ridges persist, irrespective of the stage of wear attained. 
Internally, a groove, less deep than in the molars, divides the lingual 
aspect of the tooth-crown into a pair of crescentic lobes, of which 
the anterior is the smaller. The imbrication of the premolars and the 
succeeding molars is opposed to that of the incisor-canine series. The 
crown-pattern produced by wear is a shallow basin overshadowed 
externally by a high cusp marking the termination of the second of the 
external ridges, preceded by a smaller cusp at the termination of the 
anterior ridge, the deep notch between being produced by the external 
groove mentioned above. There are no specimens in the collection 
with unworn premolars, but in a young individual retaining the milk 
dentition (No. 9482 American Museum, PI. V, figs. 11-14*7) the first 
premolar, which is rootless and probably to be interpreted as belonging 
to the permanent series, is but little worn, showing two internal crescents, 
of which the anterior is the smaller and less perfect. With the exception 
of the deep groove inclosed between the anterior horn of the smaller 
inner crescent and the anterior margin of the ectoloph, and the prominent 
ridges formed by the same parts, the outer surface of the tooth is broadly 
concave. The molars decrease in size posteriorly, but are otherwise so 
much alike that all may be described together. Antero-externally, the 
ectoloph is rendered slightly undulatory by two ridges corresponding in 
position to the external ridges already described in the premolars, but 
less distinct and almost disappearing in worn teeth. Internally, the crown 
is divided by a reentering fold into a pair of approximately equal lobes. 
On the margin of the ectoloph a series of cuspules is developed as a 
result of contact with the teeth of the lower jaw, each ridge on the 
ectoloph terminating in a cusp. A third cusp marks the junction of the 
posterior plane surface of the ectoloph with the triturating surface and in 
the third molar the elongated postero-external corner terminates in a 
fourth cusp. In No. 9482, American Museum, the third molar had not 
yet become functional. That of the left side (PI. V, fig. 11 b\ text fig. 
5, A ) shows a broadly convex outer wall and two internal crescents joining 
the ectoloph anteriorly and overlapping posteriorly, giving rise to the 
deep internal fold. The central portion of the anterior crescent is partly 
united with the outer wall by a crista-like fold (text fig. 5, A, c). The 
margin of the ectoloph is serrate, with prominent anterior and less dis- 
tinct posterior cuspules. The former persists as the cusp at the termina- 
