THE TEETH OF MAMMALS 125 



but absent on the other, so that wear produces a 

 sharp edge. Dense, horny plates are placed far- 

 ther back on the jaw, under which rudimentary 

 teeth are found, and beyond these are molars of 

 dentin and cementum. The Manatee has thirty- 

 six teeth, with square crowns with transverse 

 ridges. The upper molars have three roots and 

 the lower two. There is a deciduous premaxillary 

 tusk in the Manatee, but no incisor teeth in the 



Fig. 40. — Teeth of Dugong. 



adult, and horny plates are supplied instead. 

 Stellar 's Eytina was a sea-cow that has become ex- 

 tinct within historic times. The last of the species 

 inhabited Behring's Island. The dentition con- 

 sisted mainly of horny plates which were used in 

 cutting soft sea-weed. Some extinct forms of this 

 family are found which suggest relationship with 

 the tapirs, and it is probable that the ancestors of 

 the Sirenia were connected with the land her- 

 bivora. 



