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: UK [NCISORES; 61

OF THE INCISURES.
The Incisores are situated in the anterior part of the Jaw ; (>?)
the others more backwards on each side, in the order in which
we have named them. The bodies of the Incisores are broad,
having two flat surfaces, one anterior, the other posterior.
These surfaces meet in a sharp cutting edge. The anterior sur-
face is convex in every direction, and placed almost perpendicu-

(rf) It has been already stated that in scientific nomenclature the
term incisor is restricted to those teeth which are implanted in the pre-
maxillary hones, and in the corresponding part of the lower jaw. In the
Mammalian series, (1) if we commence with the order Bruta, we find many
instances of the total absence of teeth thus implanted. This is the case
in the Phyllophagous Bruta—the Sloths or Tardigrades; and amongst the
Insectivorous Bruta, in the Cape Ant-eater {Orycteropus Capensis), and in
the Great Armadillo (Priodon gigas). In the Armadillos of the sub-genus
Euphractus, however, a single tooth which resembles the succeeding
molars is implanted in the premaxillary bone ; and in the lower jaw the
two anterior teeth being in advance of the premaxillary tooth, are also to
be considered incisors. A portion of the lower jaw of the great extinct
Armadillo (Glyptodon) shows that the teeth extend close to the symphysis,
a corresponding implantation in the premaxillaries above being thereby
indicated.
The premaxillary bones are generally toothless in the true Cetacea : the
Dolphin, however, is an exception, one pair of its numerous teeth being
premaxillary. The Narwhal (Monodon monoceros) may be also consi-
dered another exceptional instance. In the female Narwhal, a tusk is
concealed in each premaxillary bone at its junction with the maxillary,
but remains throughout life an abortive germ; in the male, the tooth on
the left side continues to grow, and is developed into the so-called "horn,"
acquiring a length of nine or ten feet.
In common with many Bruta, the Carnivorous Cetacea exhibit an infe-
riority in their dental system not only in the acquisition of but a single
set, but in the uniformity of shape which characterises the dental
organs ; but on turning to the Aquatic Pachyderms, designated by Cuvier
Herbivorous Cetacea, we find a higher type of dentition attained. The
Dugong (Halicore) has incisors distinguished both by shape and position
from the molars ; in the upper jaw are two deciduous incisive tusks

(1) In compiling the information contained in the above and following
notes on the Comparative Anatomy of the TWtli in Mammalia, the Editor has
been chiefly indebted to the writings of Prof. Owen, F.R.8., especially the
article 'Odontology,' in the last edition of the 'Encyclopaedia Britannica.'
I