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TEETH OF THE VERTEBRATA. 489

In the African elephant {Loxodon africanu.s) the dental formula is
I. ^, C. ^, Pm, and ]\I. |-. The two incisors are greatly enlarged,
inn)lanted in deep sockets, and grow from persistent pulps. They are
preceded by small deciduous teeth, and when first protruded are tipped
with enamel, which soon Avears off. The tooth then consists mainly
of dentine covered by a thin layer of cement, the dentine presenting
a slightly modified form known as " ivory." This substance, as is well
known, is extensively used in the arts and has a fixed commercial value.
Although not exclusively confined to the tusks of the elephant, never-
theless the chief source of supply of this material is derived from them.
Tomes cites an example in A\hich a pair of tusks of this species were
exhibited in England that Meighed three hundred and twenty-five
pounds and measured eight feet six inches in length and twenty-two
inches in circumference ; the average weight, however, does not exceed
from twenty to fifty pounds. The female of this species has tusks quite
as large as the male, but in the Indian species the tusks of the male
exceed those of the female in size.
The molar teeth of the living elephants are very much alike in gen-
eral pattern and mode of replacement, which is unique ; the description
of one will therefore suffice to convey an intelligent understanding of
the entire subject.
Both existing species have a molar formula of f , which are divided
into milk molars , true molars There is sometimes, in addition to
f f .
these, a small rudimentary milk molar in front, which increases the
total number to seven upon either side in each jaw.
Although the total number of molars is normal or nearly so, they are
not all in place nor in existence at the same time. Barring the occa-
sional rudimentary one, the first molar in the Indian s])ecies cuts the
gum at a considerable distance from the front of the jaw about the
second week after birth. It is implanted by two fangs, and disj^lays
a subcompressed crown bearing four cross- ridges, and is therefore lopho-
dont in pattern. The upper tooth corresponding to this one cuts the
gum a little earlier, and possesses five cross-crests. These teeth are shed
at about the age of two years.
Before the disappearance of the first tAvo teeth the second molars
come into place from behind. They are considerably larger than the
first, being on an average two and a half inches in length by one inch
in breadth. Their crowns are of similar form, but have the number of
cross-ridges increased to eight or nine. They are implanted by two
fangs, and are shed before the beginning of the sixth year.
Bv the time the second molar has been worn out the third molar,
averaging four inches in length by two in breadth, makes its appear-
ance. Its crown has from eleven to thirteen cross-plates on its working
face, and is also supported by two fangs, of which the posterior is much
the larger. It is said to be Avorn out and shed about the ninth year.
The teeth so far enumerated are taken to be homologous with the
second, third, and fourth milk molars of the ordinary diphyodont den-
tition, which have in this case failed to develop permanent successors.
This conclusion is rendered reasonably certain, as we shall presently see,
by the fact that their ancestors had a more or less complete permanent