at one time, since it curves round the side of the head ; only enough of it therefore has been 

 drawn to show that its character is the same in all three "species". (In some old specimens 

 it exhibits a doublé row of papillae). Since then the other criteria fail to separate them, it 

 remains to be shovvn that the numbers of jaws and teeth are also inadequate for this purpose, 

 and that the "species" can be made to grade into each other. This will be sufficiently obvious 

 from the usual table of observed specimens, in vvhich, although specimens of the typical 

 formulae can readily be found, I have not found it possible to group the remainder round 

 these with reasonable sharpness. Formulae of two "species ' may be found even in the same 

 individual : for example, a specimen of which part was drawn showed a inagna formula on 

 one side (8:4: 3), and a tricuspidata formula on the other (8:2:1). Formulae: — 



