THE TEETH OF VERTEBRATES 67 



little, and each group must be traced back to the 

 original center in order to show their relation 

 between the different groups. In the study of the 

 teeth, it seems as if radiation has played an im- 

 portant part. Beginning with the primitive cone, 

 we have three theories for the evolution of the 

 molar, each of which has probably played a part 

 in the animal kingdom. Some of the animals 

 have probably had their molar teeth follow one 

 plan of evolution and some have followed another. 



The Cingulum Theory. This is one of the old- 

 est, if not the oldest theory of tooth evolution. 

 It is based on the fact that all primitive teeth 

 show a tendency to form a ridge that develops 

 near the gingival margin of the tooth, or at the 

 neck, which is a ridge of enamel on the teeth of 

 the higher mammals. The cingulum theory is 

 that upon this ridge developed small cusps or 

 cingule which increased in size in later genera- 

 tions and resulted in the extra cusps. The 

 strongest evidence in favor of this theory is that 

 we find a great many cingule on the teeth of fos- 

 sils and upon the teeth of some of the modern 

 animals. There is very little proof that the 

 molars of modern mammals are evolved in this 

 manner. 



The Concrescence Theory. This theory is 

 based on the fact that the reptiles and fishes, pos- 

 sessed a great many teeth and it has been proven 



