268 



COMPARATIVE ZOOLOGY 



two per cent of animal matter. It consists of six-sided 

 fibers set side by side, at right angles to the surfaces 

 of the dentine. Cement closely resembles bone, and is 

 present in the teeth of only the higher animals. 



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

 ber, size, form, structure, position, and mode of attach- 

 ment 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 

 only mammals have fangs. 



The teeth of fishes present the greatest variety. In 

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

 fish {Myxine) has a single 

 tooth on the roof of the 

 mouth, and two serrated 

 plates on the tongue; while 

 the mouth of the pike is 

 crowded with teeth. In 

 some we find teeth short and 

 blunt, in the shape of cubes, 



FIG. 230. jaws and Pavement teeth of a or prisms, arranged like mo- 



saic work. Such pavement 



teeth (seen in some rays) are fitted for grinding seaweed 

 and crushing shellfish. But the cone is the most common 

 form : sometimes so slender and close as to resemble 

 plush, as in the perch ; or of large size, and flattened like 

 a spearhead with serrated edges, as in the shark ; but 

 more often like the canines of mammals, curved inward 

 to fit them for grappling. In the shark, the teeth are con- 

 fined to the fore part of the mouth ; in the carp, they 

 are all situated on the bones of the throat ; in the parrot 

 fish, they occupy both back and front ; but in most fishes 

 the teeth are developed also on the roof,' or palate, and, 

 in fact, on nearly every bone in the mouth. They seldom 



