MAY, 1921. AMERICAN MARSUPIAL, C.ENOLESTES OSGOOD. 133 



involved, only one hypsodonty needs to be accounted for. This may 

 well be regarded as a recent specialization for it is of a very peculiar 

 nature, a sort of half -hypsodonty confined to one side of the teeth and 

 not fairly comparable with the full hypsodonty of the most advanced 

 diprotodonts. 



The undifferentiated median upper incisors of peramelids and the 

 supposed retrogressive canines and premolars, in the light of a study of 

 Coenokstes, do not seem at all unfavorable to a hypothesis that a dipro- 

 todont dentition might have proceeded from the peramelid stem. The 

 incisors, while not differentiated for seizing as in the dasyures and 

 didelphids, are unquestionably in a secondary condition of which no 

 further development is seen elsewhere, unless it be among diprotodonts. 

 These incisors have become broad and somewhat bladelike with their 

 cutting edges in the same horizontal plane as those of the adjacent 

 lateral incisors. In fact they are practically indistinguishable in shape 

 from the pair of lateral incisors situated next to them. Hence it seems 

 that the same factors which have led to modification of the lateral teeth, 

 may have affected the median ones. The principal of these factors is the 

 functional interaction with the lower incisors. In Perameles the three 

 pairs of lower incisors are collectively so modified in form and especially 

 in position that their functional relation to the upper incisors is much 

 like that of the elongated single pair of lower incisors in diprotodonts, 

 certainly much more so than in any other polyprotodont. They are set 

 so closely together that their cutting edges are practically continuous. 

 In addition they are exceedingly proclivous with their roots almost as 

 near the long axis of the mandible as in diprotodonts. Thus the func- 

 tional effect of the three incisors in Perameles is practically the same as 

 that of the one in diprotodonts and it is probable that specialization in 

 the lower jaw has conditioned that in the upper rather than vice versa. 



Another interesting character correlated with the proclivity of the 

 lower incisors in Perameles is found in their exposed roots, those of the 

 first and third especially being without bony covering in front. The 

 second incisor is more fully rooted, but the other two, although well- 

 developed, are very weakly attached to the jaw. Perhaps enlargement 

 of any one of the three might lead to the loss or reduction of the others 

 which would result in a diprotodont dentition. This is uncertain, but it 

 is probably fair to assume that the proclivity and the exposed roots of 

 these teeth are indications at least of some instability, possibly following 

 the loss of the fourth pair or foreshadowing a changed relation of those 

 remaining. The reduction that has already taken place in Perameles 

 corresponds to the reduction in the diprotodonts, that is, the first pair 

 of lower incisors has been lost and thus one of the stages preliminary to 



