nutrition proper. 



309 



interesting as examples of magazines of teeth of all sizes, 



and a successive dropping of old and a coming forward 



of new to their place. In a shark's jaw there is a 



magazine of many 



hundreds of teeth, 



growing smaller as 



we pass inward 



from the edge of 



the jaw (Fig. 195). 



Only the large 



teeth of the outer 



rows are in Use Fig. 195.— Section of lower jaw of a shark, show- 



at one time- but ing the magazine of teeth. The arrows show 



' the direction of replacement. 



there is a contin- 

 ual growing outward of the gum, carrying the teeth with 

 it in the direction of the arrow. Sharks' teeth are very 

 interesting in another respect— viz., as showing their 



Fig. 196. — Jaw of Port Jackson shark Fig. 197. — Jaw of a skate {Hylo- 

 {Cestraceon), showing pavement of dates), showing tesselated pave- 



rounded teeth. (Owen.) ment of teeth. (From Owen.) 



homology with scales, which are, of course, a skin struc- 

 ture. Every gradation from one to the other can be 

 traced over the edge of the jaw of some sharks. 



Pavement teeth also may be of several kinds, notably 

 what might be called cobble-stone pavement (Fig. 196) 



