160 J. T. WILSON AND J. 1". RILL. 



suppression or other modification of cusps. It is to be 

 remembered that it is only the series of primary cusps Avhich 

 appear to have nodular correspondents. 



It must also here be recalled that the examination of the 

 younger specimen " Delta " has shown us that the early 

 differentiation of nodules is not confined to the region to be 

 occupied by molar cusps. Not only is there a vestigial quasi- 

 nodular predecessor, " dv," to the enamel-organ "v," but 

 there are one or perhaps two rudimentary labial nodular 

 structures in relation to the neck of the enamel-organ " w/' 

 which disappear before the stage of specimen ''Beta" is 

 reached. 



It canuot be denied that these facts tend towards the 

 establishment of a doctrine which may be regarded as 

 coming under the category of concrescence theories. In 

 some sense or other it would appear that the molariform 

 successors of the deciduous vestigial series represent com- 

 pound structures corresponding to two or three of their 

 simple (probably homodout) predecessors. It is not neces- 

 sary to suppose that any ontogenetic fusion occurs. The 

 mode of development of the successional molars in our 

 younger stage is decisive against the occurrence of any 

 fusion process. But the relation of the two series in the 

 molar region caunot but be regarded as suggestive of some 

 sort of phylogenetic substitution of a small number of com- 

 pound teeth for a large number of simple teeth — a process 

 which must be reckoned as covering the fundamental idea of 

 concrescence. 



It might well be accepted that a given segment of the 

 dental lamina, whose constitutive material gave oiigin in 

 one phylogenetic phase to a discrete series of simple teeth, 

 might, in another and later one, lose its discrete or particulate 

 character. On such a supposition the same morphoh^gical 

 material would in its later history be concerned in the pro- 

 duction of a larger composite structure within whose limits 

 the individual elements of the pre-existing series might be 

 only imperfectly preserved or represented. 



