40 OREODON. 



Subsequentl}', small portions of the external faces of the inner lobes, in continuity 

 ■with fireater portions of the internal faces of the outer lobes, are left upon each 

 tooth, in the form of two crcscentic islets of enamel, occupjdng the middle of broad 

 spaces of dentine. The portions of the external faces are first obliterated, but they 

 are speedily followed by the remaining portions of the other faces ; and the teeth 

 then present only broad, quadrate, dentinal surfaces, bordered by enamel, and 

 bilobed internally and externally. (VI. 2.) 



All the premolars have a general resemblance to those of Anoplotherium, but 

 they differ in many details of structure. The fourth premolar is composed of a 

 single pair of symmetrical lobes, as are the functional three premolars of all exist- 

 ing ordinary ruminants, and the corresponding tooth of the Camel. The lobes of 

 the fourth premolar are the equivalents of one of the transverse pairs of the true 

 molars, except that they are considerably larger, and that the column at the .antero- 

 external margin is not as prominent. (II. 1, 3 ; III. 1, 2.) 



The anterior three premolars decrease a little in succession forward, and the 

 crown of each forms an irregular trilateral pyramid with a pointed apex. The 

 third premolar is broader posteriorly than anteriorly; and, in transverse section, it 

 forms nearly an equilateral triangle. The others are of more uniform breadth 

 transversely, and in section have a more elliptical outline. 



In all the premolars, the outer face is cordiform ; and in the last of them, it is 

 concave transversely ; in the *third it is less so ; and in the remaining two it is 

 convex, in consequence of the gradually increasing breadth of the median promi- 

 nence common to these and the true molars. 



The inner side of the ci'own of the third premolar presents a lobe which is like 

 the internal lobes of the true molai's; but it is aborted, and it occupies a position 

 exactly corresponding with that of the posterior half of the inner face of the 

 external lobes of the true molars. In advance of this aborted lobe, the anterior 

 half of the inner face of the tooth presents a double enamel fold, inclosing a pair 

 of culs-de-sac. 



The inner portions of the two anterior premolars present the same elements of 

 structure as the third, but in a more rudimentary condition; in the first premolar, 

 the postero-iuternal lobe, as it exists in the two premolars behind it, being almost 

 entirely obsolete. 



From mastication, the grinding surface of the fourth premolar passes through 

 the same phases as the corresponding portion of a true molar. Among our speci- 

 mens of the other premolars, there are none which exhibit the course of attrition; 

 but there are several which indicate that, at first, the wearing is greatest at the 

 postero-internal side. 



Inferior Molars.— (PL II. Figs. 1, 3 ; III. 3-G ; IV. 4 ; VI. 4-11.) As in all 

 existing ruminants, the lower true molars of Oreodon have two pairs of symmetrical 

 lobes ; and the last of the series has an additional lobe. 



In their form they bear a very great resemblance to those of the Deer, but they 

 are relatively more expanded in breadth, and the transverse pairs of lobes are less 

 oblique relatively to each other. Other and important difierences are briefly as 

 follows : — 



