122 FIELD MUSEUM or NATURAL HISTORY ZOOLOGY, VOL. XIV. 



with them are foreshadowed in the molar of Philander. As in the 

 selenoid group the quadrate form was produced by the addition of a 

 hypocone. 



It is in their relation to these theories of the origin of bunodont and 

 selenodont molars and also to some of the still more debatable theories 

 of the general evolution of mammalian teeth that the molars of C&no- 

 lestes must be viewed. The essential difference between the bunoid and 

 selenoid groups of Phalangeridae is in the shape of the outer labial cusps 

 of the upper molars, these being in the one convex in their external 

 aspect and in the other concave. Judged by this character alone, the 

 molar of C&nolestes at once classifies itself with those of the bunoid 

 Phalangerinae. Among these, however, it stands out as a relatively 

 primitive type. This is evidenced by (a) the high lanceolate labial cusps, 

 (b) the difference in height between the labial and lingual cusps, (c) the 

 practical absence of a hypocone in the third molar (d) the presence of a 

 distinct metaconule in the first, second and third molars. 1 When, in 

 addition to these characters of the molars themselves, the primitive 

 nature of the antemolar formula is considered, it is clear that there are 

 strong reasons for considering the molar of Canolestes as possibly 

 prototypal to those of the Phalangerinae. It is of course conceivable that 

 it is now becoming specialized in an insectivorous direction secondary 

 to a previous bunodont stage. This possibility is suggested by the 

 cursorial adaptations in the skeleton of Canolestes, but, as shown by the 

 Peramelidae, these may accompany relatively slight modifications of 

 molar structure. 



Assuming that the molar of C&nolestes represents a late stage in the 

 development of bunodonty, it becomes difficult to harmonize it with 

 current theories. Such a molar would scarcely be produced by the lower- 

 ing and thickening of cusps which appear in Philander. These changes 

 are only relatively slight deviations from the conditions prevailing 

 among all Didelphiidae. They are seen to some extent also in Didelphis 

 where, as in Philander, they are probably related to habits which are 

 more f rugivorous and omnivorous than those of Peramys and Marmosa. 

 It is conceivable that they might have led either to the bunoid or the 

 selenoid section of the Phalangeridae or to some of the conditions seen in 

 the Peramelidae. But they cannot be accepted as any indication of the 

 genesis of the quadrate buno-lophodont molar of C&nolestes. 



How, then, was this molar derived? Considerable evidence points 

 to a possible evolution by persistence of stylar elements rather than by 

 their reduction and total elimination. The theories of Bensley and 

 others, with the exception of Winge (1882), invariably give a minor role 



1 The interpretation of the metaconule as primitive is discussed on a later page. 



