22 GENERAL ANATOMICAL CHARACTERS 
except in cases of accidental or abnormal variation, and in the one 
remarkable instance of constant deviation from bilateral symmetry 
among mammals, the tusks of the Narwhal (Monodon), in which 
the left is of immense size, and the right rudimentary. In cer- 
tain mammals, such as the Dolphins and some Armadillos, which 
have a very large series of similar teeth, not always constant in 
number in different individuals, there may be differences in the two 
sides; but, apart from these, in describing the dentition of any 
mammal, it is quite sufficient to give the number and characters 
of the teeth of one side only. Since the teeth of the upper and the 
lower jaws work against each other in masticating, there is a general 
correspondence or harmony between them, the projections of one 
series, when the mouth is closed, fitting into corresponding depressions 
of the other. There is also a general resemblance in the number, 
characters, and mode of succession of both series, so that, although 
individual teeth of the upper and lower jaws may not be in any 
strict sense of the term homologous parts, there is a great con- 
venience in applying the same descriptive terms to the one as are 
used for the other. 
The simplest dentition as a whole is that of many species of 
Dolphin (Fig. 2), in which the crowns are single-pointed, slightly 
Fig. 2.—Upper and Lower Teeth of one side of the Mouth of a Dolphin (Lagenorhynchus) as an 
example of the homodont type of dentition. The bone covering the outer side of the roots of 
the teeth has been removed to show their simple character. 
curved cones, and the roots also single and tapering, and all alike in 
form from the anterior to the posterior end of the series, though it 
may be with some slight difference in size, those at the two extremities 
of the series being rather smaller than the others. Such a dentition 
is called Homodont, and in the case cited, as the teeth are never 
changed, it is also Monophyodont. Such teeth are adapted only 
for catching slippery living prey, as fish. 
In a very large number of mammals the teeth of different 
parts of the series are more or less differentiated in character, 
and have different functions to perform. The front teeth are 
simple and one-rooted, and are adapted for cutting and seizing, 
They are called “incisors.” The back- or cheek-teeth have broader 
and more complex crowns, tuberculated or ridged, and are sup- 
