222 



A MANUAL OF DENTAL ANATOMY. 



FIG. 95 (). 



portions of the jaw with teeth in situ have been discovered ; 

 thus fragments of the jaw of Acrodus, 

 t h e i go i a ted fossil teeth of which have 

 been compared to fossil leeches, with 

 seven teeth arranged in series, have been 

 met with. 



The Pristis, or Saw fish, so far as the 

 mouth is concerned, is in no way re- 

 markable, its teeth being small and 

 blunt, like those of many rays. The 

 snout is, however, prolonged to an 

 enormous length, and is shaped like a 

 gigantic spatula, its thin edges being 

 beset by dermal spines of large size, 

 arranged at regular intervals, and im- 

 planted in distinct sockets. These 

 dermal spines, or rostral teeth, as they 

 are sometimes termed, are not shed 

 and replaced, but grow from persistent 

 pulps ; in structure they closely re- 

 semble the teeth of Myliobates (see 

 page 82), being made up of parallel 

 denticles, in the centre of each of 

 which is a pulp cavity or medullary 

 canal. 



What use the Saw fish makes of its 

 armed snout is not very certainly 

 known, but its rostral teeth are of 

 interest to the odontologist for several 

 reasons the one that they are dermal 

 spines, having a structure all but iden- 



( J ) Rostrum and under side of the head of a small Pristis. a. Mouth. 

 6, Rostrum, c. One of the rostral teeth. 



The teeth, with which the margins of the jaws are covered, are so small 

 that they cannot be represented in this figure. 



