164 MOSTLY MAMMALS 



are replaced by vertical successors; and their mode of 

 development is so peculiar that in some cases the new tooth 

 is placed wrong way up beneath the one it is destined 

 to replace. In other instances, as in Coelodus from the 

 Folkestone Gault, successional teeth have not been ob- 

 served, and the mode of renewal is consequently still 

 unknown. Although within the limits of a single article 

 it is impossible to do more than give the crudest sketch 

 of a vast subject, yet what has been written may be sufficient 

 to attract my readers' interest to an extremely fascinating 

 branch of zoological study. 



