THE TEETH OF MAMMALS 123 



wear of the crowns produced two transverse 

 crests. 



The orders of Cetacea and Sirenia include the 

 aquatic mammalia which have been transformed 

 for life in the water. The Cetacea — the Whales — 

 have simple teeth, of blunt canine shape, or have 

 mere horny substitutes for teeth. The Baleen 

 Whales have a horny substance in the form of 

 plates triangular in outline, arranged in rows on 

 each side of the upper jaw. A row of smaller 

 plates is internal to the outer row. Each plate 

 is developed from a vascular pulp, and is of con- 

 tinuous growth. There are sometimes two hun- 

 dred of these plates, and the width of the matrix 

 is often two feet. The "whalebone" is fringed 

 and frayed out at the ends, and this fringe is em- 

 ployed by the animal to strain the small sea ani- 

 mals, which are found in the Arctic regions, from 

 the water. The whale gulps a quantity of water 

 containing the animals, and then closing the 

 mouth ejects the water through the fringe, thus 

 straining the animals out. Tooth-pulps are 

 found in the embryonic stage, but these are ab- 

 sorbed and disappear as the baleen is formed. 

 The Sperm-whales (Cachalot) have often fifty- 

 four teeth on the lower jaw, but the number 

 varies, the upper jaw being edentulous. These 

 are short and stout, of recurved form, and are 

 worn obtuse by use. In the Narwhal the teeth are 



