82 ON THE EXTINCT MAMMALIA OF 



molars, they became rapidly blunted, and were worn away much faster from the 

 point than at the sides. A dentinal tract was first exposed along the back border 

 of the crown, followed by one along the front border. A narrow tract next appeared 

 along the course of the internal ridge of the crown. The posterior tract greatly 

 exceeded the others in the rapidity of its expansion. As the tracts conjoined, the 

 pouches on the inner side of the crown became more or less isolated, and formed islets 

 before their final obliteration. 



In some skulls a narrow interval or slight hiatus exists between the first and 

 second premolars; in other specimens it is absent ; and in others the first premolar 

 crowds so closely on the third that this assumes a more than usually oblique position, 

 with the outer face of its crown directed forward and outward. 



The crowns of the lower true molars, as in other ruminants, have two pairs of 

 symmetrical lobes, with an additional or fifth lobe to the last tooth. They bear a 

 near resemblance to those of the Deer, but are proportionately shorter and wider, with 

 the interlobular spaces wider and shallower, and the surfaces of the lobes more 

 slanting. The median ridge internally of the inner lobes is less robust, and the outer 

 lobes are more tapering. The posterior or fifth lobe of the last molar is ovoid, with 

 an elliptical excavation at the triturating extremity. 



Wearing from attrition passed through the same phases in the inferior true molars 

 as in the Deer, excepting that from the shallower character of the interlobular spaces, 

 the bottoms of these appeared much earlier as crescentic enamel islets on the 

 exposed dentinal surflices. The islets were finally obliterated, leaving broad dentinal 

 surfaces bordered with enamel. 



The lower premolars, three in number, bear less resemblance to those of the Deer 

 than the true molars do to the corresponding teeth of the latter. They are con- 

 structed after the same plan, but decrease in size and degree of development from 

 last to first. Their crown is a broad trapezoidal pyramid, widest behind, and with 

 an acute crescentoid border rising in a median point. From the latter an oblique 

 ridge descends internally, and in the third premolar terminates in a large, trilateral, 

 pointed tubercle, which springs from the middle of the base of the crown, and rises 

 nearly as high as the principal point. In the premolars in advance, the tubercle 

 just mentioned is nearly obsolete, and the oblique ridge appears to expand into the 

 base of the crown. Back of the oblique ridge the crown presents a fossa, more or 

 less closed internally by a tubercle or ridge. In the third premolar the fossa is quad- 

 rate, and widest transversely ; in the second it is more square ; and in the first is 

 less well-defined and narrow, or appears as a mere concave slope of the back portion 

 of the crown. In advance of the oblique ridge mentioned, the inner part of the 

 crown forms a broad sloping concavity, usually enclosed at bottom by a narrow 

 festooned basal ridge. A ridge of the same character likewise exists at the back of 



