78 MAMMALIAN DENTITION 



history through less specialized members of the family the 

 method by which the molars have attained their present form 

 is clearly demonstrated. In the upper teeth the protocone re- 

 mains large, the paracone is small and what at first appears 

 to be this cusp is really part of the parastyle (style b. Bens- 

 ley). A remarkable and almost unique feature is the migration 

 of the metacone toward the palate so that this cusp comes to lie 

 immediately behind the protocone except in the last molar 

 in which it retains its normal position. The fourth large cusp, 

 occupying what is normally the situation of the metacone upon 

 the crown, is really the mesostyle (style c. Bensley). Unlike 

 the condition found in the carnivorous dentitions just described 

 the metastyle is eliminated and the metacone has taken on a pestle 

 action somewhat similar to that of the protocone. This peculiar 

 conduct of the metacone is the single known instance of migration 

 of a cusp in the teeth of modern Mammals and as such it is 

 worthy of note. Only in one order (Amblypoda) is a similar 

 condition found even among Tertiary Mammals. Migration of 

 cusps in animals of Trituberculate origin is therefore of the 

 rarest occurrence and this is significant in relation to Gregory's 

 presentation of the "Wedge Theory (See page 62). 



In the lower molars there are tAvo striking features, great 

 increase in size of the talonid and the reduction of the trigonid. 

 The former is clearly a grinding adaptation, the latter the 

 natural result of an increasingly herbivorous diet. In the 

 talonid which is raised as high as the trigonid there are two 

 cusps, the entoconid and hypoconid except in the last tooth. 

 The talonid fits itself as a mortar for the pestle-like action of 

 the protocone. In the trigonid, although the paraconid is lost, 

 there still exists a part of the shearing ledge running from the 

 site where this cusp once lay to the protoconid: this with the 

 two remaining cusps of the trigonid bounds a second mortar- 

 like area against which occludes the metacone of the upper 

 tooth next in front. 



Yet another point should be observed in the molars of Thala- 

 comys. The grinding of roots and other vegetable substances 



