96 Bidletin American Museum of Natural History. [Vol. XIV, 



the mesostyle, leaving the pre- and post-fossettes continuous 

 after all other connections have been formed. Thus a continuous 

 irregular wall of dentine is formed entirely around the tooth 

 leaving a deep valley or inlet on the internal face of the crown and 

 two large fossettes or lakes separated by the narrow transverse 

 wall of the metaloph. The outside wall of dentine is bounded 

 externally by a thin sheet of enamel, as are also the isolated 

 fossettes. Just below the point where all these ridges and cusps 

 are well united, appear the most elaborate enamel foldings, of 

 the fossette borders, that are to be seen at any stage of wear. 



From this point, which is usually about one-half to three-quar- 

 ters of an inch below the face of the unworn crown, the triturat- 

 ing surface presents a gradually less complex pattern of enamel 

 folding as the tooth crown is worn away, until in the very much 

 worn tooth the simplest pattern of enamel folding is presented. 

 Thus it may happen that a tooth will present the most elaborately 

 complex enamel foldings when it is little worn, and become most 

 simple in this respect when in old age the crown is worn very 

 short, exhibiting all the intermediate stages in the course of wear. 

 This principle is demonstrated in Fig. 3. A represents the little 

 worn crown (No. 2726, Nebraska Coll.) of a fossil tooth of 

 moderately complicated pattern, A\ A^, and A^ are three sections 

 of the same tooth, and A* indicates where each of these sections 

 was cut. Note the difference in general of the fossettes and 

 also the changes in the character of the outside enamel foldings. 



(II), Effects of Wear on the Proportions of the Teeth. 

 I, The Teeth Taken Individually. 



Unlike the degree of complexity of the enamel foldings, the 

 corresponding diameters are affected differently by wear in differ- 

 ent teeth of the molar-premolar series. The same general rule 

 for the change in ratio of the antero-posterior to transverse diam- 

 eter may be applied to the intermediate teeth p^ to m^ in- 

 clusive, but the most anterior and posterior teeth (p^ and m^) 

 are affected differently, in this respect, from the intermediate 

 teeth of the series and from each other as well. 

 a. Laws Governing the Changes of Diameters of the Tooth Crowns. 



There seems to be no exception to the following laws for the 

 changes of diameters of the tooth crowns as they are worn away 

 by use. 



