368 MICROSCOPIC ANATOMY OF THE TEETH 



and embedded in the horny plates, and persist for a con- 

 siderable portion of the life of the animal, and ' are only 

 shed after being worn down by friction with food and sand '. 

 Dr. Semon, who has studied the habits of the Ornitho- 

 rhynchus, says that it feeds chiefly on ' grubs, worms, snails, 

 and, most of all, mussels ', which are stowed into its cheek 

 pouches, ' the food being chewed and swallowed above the 

 surface as the animal drifts slowly along '. He considers 





FIG. 243. Vertical section of developing tooth of Petromyzon marinus, 

 showing a successional tooth which is just beginning to cornify at its 

 apex beneath the functional tooth, d. Dermis ; dp. dental papilla ; 

 ep. epidermis lining buccal funnel ; ep l . epidermis which has formed the 

 horny functional tooth ht ; ep 2 . epidermis forming the horny cone of the 

 successional tooth ht 1 . (From Warren.) 



that for cracking the hard shells of the mollusc upon which 

 it mainly feeds, the horny plates are preferable to brittle 

 teeth (6). 



The eight or ten molar teeth which form the calcined 

 dentition of the animal are replaced by the horny structures, 

 which, developed from the epithelium of the mouth, are 

 produced around and under the true teeth, embedding them, 

 the short roots of the teeth passing through them to the 

 bone of the jaw. As Dr. Beddard says, ' The epithelium of 



