GENERAL CHARACTERISTICS. 
11 
only one side of each jaw, and indicating the incisors by the letter i, the canines 
or tusks by c, the premolars by p, and the molars by m, and taking the num¬ 
bers above the lines as representing the teeth of the upper, and those below the 
<same the teeth of the lower jaw, we may express the number and kinds of the 
teeth of the dog by the 
formula : i\, c \ , p|, m|. 
The total thus given is 
21, and double this num¬ 
ber will of course give 
the entire number of 
teeth on both sides of 
the two jaws, which in 
this case will be 42. 
Structure of A few 
the Teeth. words must 
now be said regarding 
the internal structure of 
teeth, as without this it 
IS quite impossible to rp 0 s h ow distinction 'between incisors, tusks, and cheek-teeth.—After Nehring. 
understand the modifica¬ 
tions which they undergo in different groups of Mammals. Taking a simple 
more or less conical tooth like the tusk of a lion or tiger, or any tooth of a 
sperm whale, it may be observed that when such a tooth first appears above the 
gum it is open at the base, where it forms a hollow cone. And in teeth like the 
tusks of the elephant, which grow throughout the whole life of their owner, such 
a condition remains permanent. Usually, however, a tooth ceases to grow after a 
certain period, and the base of the root or roots then becomes completely closed, 
and assumes a pointed shape. A tooth of this simple conical type is composed 
internally of a substance known as the ivory or dentine, coated externally with a 
thin layer of a much harder nature and highly polished appearance, which is termed 
the enamel. Moreover, outside the base of the crown there may be patches of a 
coarser substance, called the cement. A model of such a tooth may be made by 
taking the finger of a kid glove, filling it with bees-wax, and putting some smears 
of sealing-wax at the base of the outer surface, when the bees-wax will represent the 
ivory, the kid the enamel, and the sealing-wax the cement. If we then cut off the 
summit of the finger we shall have a central disc of bees-wax (ivory) surrounded 
by a circle of kid (enamel), which will represent the condition of such a type of 
tooth when its summit has been worn away by use against the opposing tooth of 
the opposite jaw. If, however, before cutting off the end of our model, we indent 
the summit with several deep pits, and also mark the sides with one or more 
grooves, and fill up such pits and grooves with sealing-wax, it is obvious that we 
shall have a much more complex type of structure. This complex model will serve 
to explain the type of tooth structure found in many of the Hoofed or Ungulate 
Mammals; and it will be obvious that if we now cut off the summit of our model 
we shall find a series of irregular discs of bees-wax (ivory), each surrounded by a 
sinuous border of kid (enamel), in the folds of which will be masses of sealing-wax 
SKULL OF PIG. 
