way from the fully mature foetus of the female No. 1, except that its teeth are erupted,
the thirteenth tooth on the left side of the upper jaw is 0*22 inch in antero-posterior
breadth at its base, and 0‘35 inch long, whereas in the female No. 1 and in the mother
of the foetus the same tooth has a basal extension of not less than 0'87 inch and a
length of 0‘90 inch ; in other words, the tooth grows from youth to age, but the nature
of this growth will be seen hereafter. Eschricht described the arrangement and alternation
of the teeth, the latter feature being occasionally carried to such a degree that the
most anterior of the small conical teeth of the two sides, especially in the lower jaw,
appear as if arranged in one line. Eschricht regarded the alternation of the teeth
as a means seemingly to get room ; but may it not rather be due to the, so to
speak, natural tendency of the skull to asymmetry, because it is associated as a
rule with an unequal number of teeth on either side of the jaw ? Eschricht hqs
also described the general characters of the teeth in youth ; but it will be observed
(Table IV) that the teeth in the lower jaw are not, invariably at least, equal to
those in the upper, viz., twenty-nine on each side, as observed by Eschricht, because
in Nos. 3, 6,12, and 14, this rule does not hold good : the teeth, however, are generally
more numerous in the lower than in the upper jaw. Notwithstanding the greater
number of teeth in the lower jaw, the two or four posterior upper teeth extend
behind them and have no teeth apposed to them below (see Pl. XXXIX), as observed
by Eschricht.
The lower are much longer than the upper teeth, and when in position in youth
they interlock between the latter, overlapping the sides of the upper jaw, the teeth
of which also, to a more limited extent, are applied at their points to the outside of
the mandible, and the teeth are in such close juxtaposition that the anterior teeth
rub against each other, and these, as Professor Owen1 has observed, retain a prehensile
structure contrary to the rule in Delphmidæ.
The first tooth of the lower jaw is placed anterior to the tooth of the upper jaw.
The first two or three teeth are convex anteriorly and concave posteriorly, but the
long teeth immediately succeeding these are flattened on their anterior and posterior
surfaces, slightly upwardly and inwardly curved in the lower jaw, and the reverse
in the upper, the external surface of the teeth being convex. The points in youth
are very sharp. The long teeth diminish after the eighth rather rapidly.
In the young state the alveolar groove is not divided into compartments or
sockets for the reception of the teeth, but is an uninterrupted furrow, from end to
end, in which they are lodged. The inner margins of the groove, however, are wavy,
and the little inward sinuosities indicate the commencement of osseous ridges,
which increase with age and which ultimately, and while the skull is yet young, are
converted by osseous growth into transverse partitions, which divide the alveolar
groove into a number of well-defined sockets, in each of which is lodged a tooth. In
the young state, the pulp cavity in the dried skull occupies the lower half of the
tooth, but this cavity becomes closed at an early age and while the exposed portion
of the tooth yet retains the characters of youth. The bases of the teeth while the
Odont. p. 362, Cat. Ost., 1. c., p. 449.
pulp cavity is still open are laterally compressed, the longitudinal being about twice
as long as the transverse diameter of the orifice; but when the cavity is closed, the
base of the tooth longitudinally is so much laterally compressed as to assume the
character of an osseous plate, carrying obliquely on its apex the portion of the tooth
which originally appeared beyond the gum, and retaining all its youthful characters
and with an unworn tip, but with its anterior border in the case of the lower teeth
deeply grooved by the friction of the upper tooth apposed to it, the latter teeth
becoming similarly worn on their posterior borders.1 Associated with this closure
of the pulp cavity occurs an increase in the dimensions of the base of the tooth.
Fig. 13.
Series o f teeth o f Platanista gangetica, to illustrate their growth and wearage. A ll drawn o f natural dimensions.
1—One of the long sharp front teeth of a young animaL
2—A tooth from the posterior end of 'the row of lower jaw of a young animal.
3—An adult tooth where wearage of crown is most irregular, and only a small piece o f the enamel remains: pillars
of cementum are produced a t the fang. .
4—Shows an adult tooth where great widening and flattening of the fang has taken place, the crown heing still covered
with enamel, and long and conical in shape.
5—Another adult tooth equally showing increased flattened root growth, hut diminishing crown; a cap o f enamel still
exists, but the dentinal substance immediately below is highly polished on the outside, the pearly lustre simulating
g 0^ er more worn tooth, where the crown rubbed down to a low nipple shape is destitute of enamel, and even the
dentine is altered into the so-called osteodentine.
7—A tooth now almost crownless, and the body a mere strip uniting the columnar superadded root of cementum.
8—A further stage where even absorption of the false root has proceeded, while the element? of the original tooth itself
are not only rubbed down, but utterly gone: what remains is pure bony substance, substituted, but functionally
answering for a dental organ.
In a more adTaneed stage the jaws are much deepened, and osseous deposits are
observed to have taken place in the sockets, the jaw at the same time growing in
all its dimensions, the bases of the teeth having noticeably increased in sfae, while
the free original portion has become worn away on its sides and reduced in length
by wear and breakage. At this stage, the teeth are not so deeply set in their sockets,
a ciroumstance due to the fact that they seem to he outwardly pushed by the accumulation
of osseous growths in the sockets themselves, and by their own enlargement.
With increasing age the portions of the teeth that originally pierced the gum
are worn away entirely, until nothing but their enlarged bases remain, the free ends