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488 DENTAL ANATOMY.

development of a long diastema in front of the premolars. While
the complete assumption of these characters is reached only in the
bovine ruminants, others exhibit all the intermediate stages of modi-
fication tending in that direction.
The common Virginia deer (Cariacus virginianus) has been selected
as an average example of the higher selenodont dentition ; although
in its family (Cervidte) canines are sometimes found in the upper
jaw, there is little or no cementura on the crowns of the molars, and
they have well-defined roots. It will therefore be observed that it does
not fulfil all the requirements* of the most highly specialized selenodonts
in its dental organization. The dental formula of this species (Fig.
265) is I. -|, C. {, Pm. |, M. | = 32. The incisors have long
spatulate crowns, the median pair being the larger, the outer ones
decreasing gradually in size. The canines are smaller than the outer
pair of incisors, which they resemble very much in shape, being applied
closely to them. After an immense interval follow the premolars, the
first two in the lower jaw being comparatively simple, the third four-
lobed like the succeeding molars. The molars display two perfect
double crescents, of which the outer are convex externally. The last
molar has a fifi:h lobe. In the upper jaw the premolars are bilobed,
the internal being convex internally and enclosing a deep valley between
it and the external cusp. The true molars have double crescents enclos-
ing two valleys. The antero-internal of these crescents is made up of
the anterior intermediate tubercle, which has become greatly enlarged
and developed into a crescentic form, the true antero-internal cusp being
situated internal to and behind it. The proper evidence to support this
determination is to be found by examining the superior molars oi Hyopot-
amus, Anoplotherium, and Xiphodon, it which it will be seen that the
antero-internal cusp becomes gradually smaller.

Teeth of the Proboscidea.
The last order of the ungulate series whose dental organs remain to
be noticed is that including the elephants, mastodons, etc. The animals
composing this group are the largest of terrestrial mammals, and display
many curious modifications of the primitive ungulate tyjie. Probably
no ]mrt of their organization has been more profoundly affected in their
gradual evolutionary growth than the teeth, and were it not for the fact
that abundant evidence is at hand to demonstrate the successive steps in
the progressiv^e modification from a more simple type, we would be at
a loss to comprehend tiie manner of production of these most complex
of all teeth.
Two genera of ]iroboscideans are fi)und in the existing faunae of Asia
and Africa, but tliese are only tlie inconsiderable remnant of a once
greater and nnich more widely distributed representation, as is indicated
by their fossil remains. During the later Tertiaries proboscideans were
not unknown in both the northern and southern hemispheres in all the
extensive land-areas ; in some parts of the northern hemisphere, where
thev are now extinct, judging from their fossil remains immense herds
and droves must have at one time existed.