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 correspondingdepressions 

 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 



Fio. 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- 



