MAY, 1921. AMERICAN MARSUPIAL, CENOLESTES OSGOOD. 123 



to the styles, although they are so highly developed in the tritubercular 

 molar. They are supposed to grow up from the cingulum and then to 

 shrink back into it to complete extinction. In some cases this may 

 occur, especially in placentals, but in the marsupials there is much to 

 show that the styles have played an important part and that they may 

 be the principal elements preserved in the outer cusps of the molars of 

 C&nolestes and the bunoid Phalangerinae. In some cases they may 

 have wholly supplanted the paracone and metacone and in others they 

 may have fused wholly or partly with them, the resulting cusp usually 

 preserving more of the character of the style than of the so-called 

 primary cusps. This hypothesis favors Winge's theory that the styles 

 are the original elements of the molar crown persistent from the tri- 

 conodont stage, but it is only facts bearing on the evolution of the 

 bunoid molar from the tritubercular that are of present importance. 

 Some of these are the following: 



1. The stylar elements of the molar crown are deep-seated struc- 

 tures, being recognizable in many early mammals and in developmental 

 stages of recent ones. 



2. They occupy the outer or labial side of the teeth which is phylo- 

 genetically older than the inner or lingual. 



3. They are well developed in polyprotodont marsupials and show 

 a high degree of consistency in general relations (Bensley, 1906). 



4. Morphologically they are bunoid, at least conical or compressed 

 conical in section. 



5. The cusps of the trigon in the tritubercular molar are selenoid, 

 essentially alike and essentially different from the styles. 



6. Hence the crown of this molar consists of two elements one of 

 which is fundamentally bunoid and probably primitive while the other 

 is selenoid and secondary. 



7. The styles are situated directly over the buccal roots and in line 

 with the primitive longitudinal axes of the premolars in the identical 

 position of the outer cusps of the quadrate bunoid molar. 



8. On the contrary, the paracone and metacone are midway between 

 the lingual and buccal roots and in order to become the outer cusps in 

 the bunoid molar would require not only a change in form but a migra- 

 tion from their position. 



9. Aside from the confusing factors introduced by minor adapta- 

 tions, therefore, the general relations in the tritubercular molar are 

 favorable for positive participation of stylar elements in the formation of 

 the bunoid molar. 



With these broader facts in view, evidence may be sought as to 

 detailed methods of change other than that of downward reduction 



