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. 
