40 OREODON. 
Subsequently, small portions of the external faces of the inner lobes, in continuity 
with greater portions of the internal faces of the outer lobes, are left upon each 
tooth, in the form of two crescentic islets of enamel, occupying 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 Anoplotheriwm, 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. (IL. 1,5; 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 inney, side of the crown of the third premolar presents a lobe which is like 
the internal lobes of the true molars; but it is aborted, and it occupies a position 
exactly corresponding with that of the posterior half of the inner face of the 
external Jobes 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-internal 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.—(P\. II. Figs. 1,3; I. 8-6; 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 differences are briefly as 
follows :— 
