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TEETH OF THE VERTEBRATA. 397

the teeth into several categories, the convenience of which, to say the
least, if not the real importance, is undeniable. The question of the
nature and relationship of the milk dentition to the permanent one
will be discussed after the teeth of the several groups have been
considered.
The Teeth and their Accessory Organs in the Dog.—I have
thought best to next present a detailed des(!ription of the adult structure
of an average diphvodont dentition, together with the accessory organs,
in order that the student may become familiar with the technicalities
before proceeding to consider the teeth of the various sections of the
Mammalia The dog has been selected as an example of this kind,
])artially on account of the generalized condition of the teeth, but
principally on account of the readiness with Avhich the student will
be enabled to provide himself with the necessary material.
The teeth of the dog (Figs. 195 and 196) are forty-two in number,
of which twenty belong to the upper and twenty-two to the lower
jaw. The most anterior teeth of the upper series are relatively small,
and are imjjlanted in the free edge of the premaxillary bones in such a
manner as to describe the arc of a circle. These are known as the
incisors {ic, Figs. 195, 196). Behind these, after a slight interval, are
Fig. 195.

ic, incisors; e. canine ; pm, premolars
Vertical View of tlie Upper Jaw of a T)og {Ornis familiaris) : ;
111, molars; .«, sectorial ; pmz, premaxillary bone; mx, maxillary bone; pi, palatine ; <(///, anterior
p ilatine foramen ; ppf, posterior p.ilatine foramen. The position of the third premolar is slightly
abnormal.
a pair of strong, laterally compres.sed curved teeth, the canines, which are
lodged deeply in the sub.stance of the maxillary bone, immediately behind
the maxillo-premaxillary suture. Behind the canines, again, are six
teeth on each side, wliich progressively increase in size and complexity
as we proceed backward until the penultimate tooth is reached, the last
one being somewhat smaller. These are termed molws and premolars.
The tooth-line of each moiety of the upper ja^v presents three curves,
the most anterior of which is formed by the three incisors and canine,
with a strong convexity outward ; the line of tlie next four describes a
gentle curve whose convex surface is inward ; while that of the la.st two
curves boldly inward toward the median line.
The number of teeth in the lower jaw is one in excess of that of the
upper, which is caused by the addition of a small single-rooted tooth at