234 A MANUAL OF DENTAL ANATOMY. 



ness of the three tissues a constant roughness of surface is 

 maintained. The upper pharyngeals are similarly armed ; 

 and as the teeth and the supporting bone wear away, fresh 

 teeth are developed at the front, so that the whole bone 

 undergoes a sort of gliding motion backwards, the armature 

 of the lower pharyngeal being renewed in a similar manner, 

 save that new teeth and bone are developed at its posterior 

 instead of its anterior extremity. 



The teeth are developed in bony crypts,beyond the youngest 

 functional teeth, and perforations in the roofs of the crypts 

 give passage to the connecting band between the tooth sac 

 and the mucous membrane. 



No more fitting place will occur for noticing the stout 

 pharyngeal teeth whicn are met with in so many fish. 

 Some fish, which are edentulous so far as the mouth is 

 concerned, have the pharyngeal bones armed with teeth ; 

 in the carp and its allies, edentulous so far as the mouth 

 proper is concerned, the two lower pharyngeal bones carry 

 long pointed teeth, which partly oppose one another, and 

 partly oppose a sort of horny tubercle, which is supported 

 on a process of the base of the occipital bone. 



A few fish are quite without teeth ; the sturgeon, whose 

 mouth forms a protrusible sucker, is edentulous, as are also 

 the pipe fish, and the little sea horse (Hippocampus), now so 

 common in aquaria. 



But as a rule fish are remarkable for the great number of 

 their teeth, which are being constantly shed off' and replaced 

 by successors an indefinite number of times. 



In all the fish hitherto mentioned in these pages, it 

 happens that the teeth in different parts of the mouth differ 

 in size and in the function which they have to perform ; 

 but this is only so because a few striking forms have been 

 naturally selected for description. It is far commoner for 

 all the teeth of fish, particularly of those fish which have 

 countless numbers of teeth, to be very nearly alike in form 



