66 COMPARATIVE ZOOLOGY. 



in rows from the gums of the upper jaw. In some Whales 



there are about 300 plates on each side. 25 



True teeth, consisting mainly of a hard, calcareous sub- 



stance called dentine, are found only in back-boned ani- 



mals. They are distinct from the skeleton, and differ 

 from bone in containing more min- 

 e eral matter, and in not showing, 

 under the microscope, any minute 

 cavities, called lacunae. A typical 

 tooth, as found in Man, consists of 

 a central mass of dentine, capped 

 with enamel and surrounded on 

 the fang with cement. The first 

 tissue is always present, while the 

 others may be absent. It is a mixt- 

 ure of animal and mineral matter 



FIG. 31. Section of Human Mo- . 



lar, enlarged: k, crown; n, disposed 111 the form of extremely 



tubes and cells, so minute as to 

 cavit y- prevent the admission of the red 



particles of blood. One modification of it is ivory, seen 

 in the tusks of Elephants. Enamel is the hardest tissue 

 of the body, and contains not more than two per cent, of 

 animal matter. It consists of six-sided fibres set side by 

 side, at right angles to the surfaces of the dentine. Ce- 

 ment closely resembles bone, and is present only in the 

 teeth of the higher animals. 



Teeth are usually confined to the jaws ; but the num- 

 ber, size, form, structure, position, and mode of attachment 

 vary with the food and habits of the animal. As a rule, 

 animals developing large numbers of teeth in the back 

 part of the mouth are inferior to those having fewer teeth, 

 and those nearer the lips. The teeth of Mammals only 

 have fangs. 



The teetli of Fishes present the greatest variety. In 

 number, they range from zero to hundreds. The Hag- 



