386 



tion, the more internal arising first. Thus the whole ribbon of dental 

 organs increases in length and breadth by additions made respectively 

 to its anterior extremity and sides ; and each transverse row gradually 

 moving backwards by the continued development and growth of others 

 anterior to it, causes elongation of the lingual sac, which only attains 

 its perfect state when these processes are at an end. The idea, there- 

 fore, that the new teeth are developed from behind forwards and suc- 

 cessively brought into use, as in sharks and rays among fishes, does 

 not appear to me to be correct." 



In the annexed Table a rough arrangement is given of a consider- 

 able number of genera grouped together by the characters above re- 

 ferred to. Although the author thinks it improbable that any genera 

 opposed to each other in those fundamental particulars can be inti- 

 mately related, yet the facts are not advanced as the basis of a new 

 classification, but simply that they may yield their own weight, as so 

 many available tests of affinity. 



