TEETH AND SPINES. 41 



in size and form. In the Greenland shark, for 

 example, they are comparatively small. In the 

 larger sharks they are either spike-shaped or 

 triangular in form, and in some fossil sharks the 

 triangular type of tooth reached huge propor- 

 tions. They form terrible weapons in many 

 living sharks ; an instance is on record where 

 a man has been bitten in two at a single bite ! 



The origin of the curious comb-like teeth, and 

 of those teeth of sharks which have a large 

 middle cusp or point with a smaller one on 

 either side, is peculiar ; being due to the fusion 

 of three or more of the primitive single teeth 

 into one. How this came about we can see by a 

 study of the coarse " shagreen " of say the spiny 

 shark (Echinorhinus). Here it will be found 

 that little groups of these tubercles, which are 

 scattered irregularly over the surface, become 

 welded or fused together at their bases here a 

 group of five, forming a long patch armed with 

 as many tooth-like spines, there a group of two 

 or three, and here and there a single tubercle 

 (fig. 4). On the jaw similar fusions have taken 

 place though they are always of denticles lying 

 side by side. As a result we get the " comb " 

 teeth of Notidanus, or the A-shaped teeth of other 

 sharks. The large plate-like crushing teeth of 

 many fossil and recent sharks, as well as of 

 fishes belonging to higher groups, have been 

 formed in this way. 



Amongst the bony fishes, of which we may 

 take the perch, pike or cod-fish as examples, 

 the variations in the form of the teeth are quite 

 as numerous as in the shark tribe. In some 



