162 MAMMALIAN DENTITION 



Lemur (Fig. 36) but if this is so the production of the cusp is 

 the result of parallel evolution. According to Bolk the extra 

 cusp occurring on the palatal side of the upper molar and on 

 the lingual side of the lower corresponds to a fourth molar and 

 is genetically different from that on the lateral aspect of the 

 tooth. 



In the lower molar the hypoconulid is found subdivided not 

 infrequently, a condition analagous to the splitting of the 

 metacone of the last upper molar seen in the Koala (Fig. 24). 

 Again the lower molars, as in Fig. 56-7, are apt to present a 

 sixth cusp on the lingual side of the croAvn between the meta- 

 conid and the entoconid. This occurs as a small tubercle al- 

 most regularly in the Gorilla and is found also in many Eocene 

 Primates (see page 144). 



Returning now to the accessory cusp on the lateral aspect of 

 the molars it should be noted that sometimes an extra tooth 

 with a simple crown occurs in the interval between the molars 

 in either the upper or lower jaw and it is impossible to avoid 

 the conclusion that the accessory cusp really represents this 

 supernumerary tooth imperfectly separated off from the normal 

 molar. The extra tooth invariably occurs in the same situation 

 but in the upper jaw, if it be imperfectly separated from the 

 permanent molar, it is represented usually as a cusp whereas 

 in the mandible it most frequently occurs as an accessory root 

 between and lateral to the two normal roots. The presence of 

 this anomaly in its various forms has furnished Bolk with 

 grounds for a theory that the first molar, in which he avers 

 such an abnormality is never found, belongs to the milk denti- 

 tion along with the supernumerary denticles just described, 

 which may or may not fuse with the second and third molars, 

 and that the second and third successional molars with the 

 occasional fourth belong rightly to the permanent set. Before 

 this vieAV is accepted, however, it must be subjected to further 

 critical examination. 



Before terminating this very rapid and by no means complete 

 revieAV of anomalies in the human dentition it is necessary to 



