172 COMPARATIVE DENTAL ANATOMY 



by two inches in size. The fourth follows in the 

 ninth or tenth year, has fifteen to sixteen plates, 

 and is eight by three inches in size. The fifth 

 molar appears at the twentieth year, has seven- 

 teen to twenty plates, and is ten by three inches 

 in size. The sixth molar appears ten to twenty 

 years later, has twenty to twenty-seven plates, 

 and is twelve to fifteen inches in length. This 

 lasts until the close of the animal's life, which may 

 be a century. These molars are very complex and 

 remarkable in structure. Each consists of a 

 series of transverse, oval plates like flattened cir- 

 cles of enamel, the central space being filled with 

 dentin, the bulk of the crown between and around 

 the plates being made of cementum, — the unequal 

 density of the tissue producing a rough surface 

 for grinding, as in all Herbivora. The occluding 

 surfaces are originally tuberculate with crown 

 crests, like the molars of the Mastodon, but these 

 soon wear down and expose the plates, which are 

 arranged transversely to resist the anteroposte- 

 rior movement of the mandible. These plates 

 present different patterns in the Indian and Afri- 

 can species; in the former they are of parallelo- 

 gram or diamond shape with zig-zag foldings of 

 the enamel rim, and in the African species the 

 plates are lozenge-shaped, or flattened circles, and 

 the rim of enamel is smooth and even. The 

 molars are composed of separate enamel plates. 



