Page 361 - My FlipBook

TEETH OF THE VERTEBRATA. 371 ; ;

upon the cartilaginous mandibuli and palato-quadrate arches, being
attached not to the cartilages themselves, but to a thick, dense fibrous
membrane Avhich forms an external investment. They are arranged in
concentric rows on the summit and inner surface of the jaws, being
developed from the bottom of a longitudinal fold of the lining membrane
in this situation, known as the thecal fold. The teeth of the upper-
most row, or those occupying the margins of the jaws, stand upright
and do service as the functional ones until discarded ; those of the next
row, as well as all the succeeding ones, usually occupy a recumbent
position, with their apices directed downward or upward according as
they belong to the upper or lower series ; but it not unfrequently hap-
pens in some species that the second, and even the third, rows may
exhibit different degrees of erection. As a general rule, but a single
row of teeth are in use at one time. The individual teeth composing
the longitudinal rows may be disposed with reference to those of the
succeeding ones so as to be parallel vertically, as is M^ell exemplified
in the genus Lamna, or they may be placed in such a manner as to
alternate with each other, a condition seen in the blue shark (Car-
charias). As would naturally be surmised from this arrangement, the
way in which succession takes place is for the row beneath to rise up
and take the place of those in use. This is accomplished by the fibrous
gum in which their bases are imbedded sliding bodily over the curved
surface of the jaws from within outwaixl, continuously bringing fresh
rows into position, as was long since demonstrated by Prof. Owen.
It thus happens, on account of this peculiar and, in my judgment,
remarkably primitive manner of succession, that large numbers of teeth
little worn are cast oif during the life of each individual, and that
replacement goes on far in excess of the actual requirements of the ani-
mal, and quite independently of their temporary use as organs of pre-
hension and mastication—a fact which in itself demonstrates their der-
mal relationship. The only assignable cause for this extravagant devel-
opment of teeth, it appears to me, is due to inequalities in the rapidity
of growth in different parts of the body, which causes the integument
invaginated during embryonic development to be restored or evaginated
during adult growth. If this hypothesis be correct, then the whole
question of the force concerned in the succession of the teeth is reduced
to the simple explanation of inequalities of growth primarily, however
much it may have been subsequently complicated and obscured in the
higher forms. Looked at from this standpoint, it is not such an
inscrutable mystery as C. S. Tomes and others would have us believe.
Considerable variety of form exists in the teeth of the different species
they may be heterodonf (that is, different in various parts of the jaws)
isodont (alike throughout) ; or hemihomodont (in which the individual
teeth of the lower jaw are alike, but different from those of the upper
jaw, and reciprocally). In all, the teeth nearest the back part of the
mouth are smaller than those in front. The simplest form to be met
with is the unmodified cone with a sharp point and a broad base. Such
is found in the large Rhinodon and some "dog-fishes;" to this may be
added basal denticles, as in the genus Lamna; or it may have a com-
pressed triangular outline with serrate edges, as in the upper teeth of