LIFE HABITS AND DENTITION 85 



most significant features is the occurrence of crescentic or 

 selenodont cusps. In the upper teeth proto-, para-, meta- and 

 hypocone are all present, the inner cusps being less prominent 

 than the outer ones and the hypocone, as a younger cusp phylo- 

 genetically, smaller than the protocone. Each cusp presents a 

 concavity open outwards. On the lateral margin of the tooth 

 occur the parastyle and mesostyle with a vestige of the meta- 

 style. Owing to the lateral movements in mastication the cusps 

 of the talonid of the lower molars swing transversely through 

 the groove between the protocone and paracone in front and 

 the hypocone and metacone behind: apparently in association 

 with this the mesostyle is bifid. The last upper molar presents 

 a noteworthy feature in having a pseudohypocone. This cusp 

 simulating the hypocone in position and appearance is observed, 

 by comparison with less specialized members of the family, to 

 be produced by the subdivision of the metacone. If ever a true 

 hypocone occurred on this tooth it has been lost and a tritu- 

 berculate condition restored as a specialization which may be 

 seen in less advanced crescent-toothed Phalangers. Later in 

 response to necessity a pseudohypocone was developed as al- 

 ready mentioned. The progress of evolution towards reduc- 

 tion of the fourth upper molar has in this case been first ar- 

 rested and then actually reversed. In other words this is an 

 instance of reversibility of evolution. In association with this 

 the last mandibular molar is only slightly reduced in size. 

 The cusps as in the upper teeth are crescentic but the con- 

 cavity is directed inward. As in the maxillary teeth the 

 crescents are more pronounced in the outer cusps. The para- 

 conid is the only cusp lost. The occurrence of a slight ex- 

 ternal cingulum diminishes somewhat the notch between pro- 

 toconid and hypoconid. 



A selenodont adaptation of molar cusps occurs several times 

 in the evolution of mammalian dentitions. It represents one of 

 the inherent responses of the tooth to functional requirements 

 and is by no means confined to a single order. 



There remains another modification in which however 



