90 COMPAEATIVE DENTAL ANATOMY 



in concentric rows on the inside and summits of 

 the mandible below and the palato-quadrate 

 arches above (Fig. 30). They are imbedded in 

 and borne upon the fibrous membrane which 

 lines the inner surfaces of the arches, and by 

 which they are carried forward over the edge of 

 the jaws. The teeth lie flat against the membrane 

 until brought to the edge, where the turning 

 brings them upright for use. Thus the first row 

 stands upright on the margin of the jaw and does 

 service until lost, then those of the next row rise 

 up to take the place of those lost, and the succeed- 

 ing rows follow in regular orders, — the succession 

 being endless. The teeth are developed from the 

 bottom of a longitudinal fold of the lining mem- 

 brane of the mouth at the lower edge of the jaws, 

 called the thecal fold, from which they are con- 

 tinually carried up over the edge of the jaws. In 

 all species the teeth are largest in the front or 

 center of the jaws and diminish in size backward. 

 The teeth of sharks are peculiarly adapted to the 

 destructive habits of their possessors. The sim- 

 plest form is that of a simple cone with a sharp 

 point on a broad base, or it may be a triangular 

 cone. In most of the sharks, however, the teeth 

 are flattened into a triangular form, sometimes 

 with serrated edges. The serrations are often 

 deep and sharp and the tooth makes a very ef- 

 fective implement for cutting and tearing. The 



