MAY, 1921. AMERICAN MARSUPIAL, C^NOLESTES OSGOOD. 113 



many as six unicuspids in the lower jaw, giving it a total antemolar 

 formula of nine, exceeding that of any living mammal. While this 

 formula, as observed in only one specimen, may be abnormal, as sug- 

 gested by Sinclair, it is evident that the formula of C&nolestes, large 

 though it be, has already undergone a reduction. A further and gradual 

 reduction may be traced in Australian diprotodonts to those in which 

 only the median or terminal pair of incisors and one pair of differentiated 

 premolars remain, two pairs of antemolar teeth in all, as in Phascolarctos 

 and Phascolomys. 



Representatives in this series with the number of antemolar 

 teeth in each would be as follows: Garzonia, g; Halmarhiphus, 8; 

 Ccenokstes, 7-8; Phalanger, 6-7 -, 1 Acr abates, 5; Dromicia, 4-5; Dis- 

 taechurus, 4; Trichosurus, 3-4; Macropus, 3; Phascolarctos, 2. When the 

 form and character of the teeth as well as their number is considered, 

 the probability that the series is an expression of homologies seems very 

 great. Among the living forms of the Phalangeridae the reduction is 

 going on at present, as indicated by the great instability of the number 

 of the "small intermediate teeth" in various species. This was shown 

 especially in the genus Phalanger by Jentink (1885) who examined a 

 considerable series and found great variation. The subject has been 

 further elaborated by Bateson (1894). In the extinct forms Pal&othentes, 

 Acdestis, and Callomenus, a reduction also has occurred, these having 

 only five or six antemolar teeth. Hence, C&nolesles is more primitive 

 than these. In fact, so far as mere number of teeth is concerned, Ccsno- 

 lestes and the allied extinct forms like Halmarhiphus and Garzonia are 

 almost as primitive as the didelphids. Assuming their common ancestry, 

 it is necessary to believe that the diprotodont modification preceded any 

 numerical reduction. Moreover, it is not impossible that all living and 

 extinct polyprotodonts are more reduced than these primitive dipro- 

 todonts. The case of Garzonia with its nine lower antemolar teeth is of 

 interest in this connection. At least five of these must be regarded as 

 incisors unless reduplication with no reference to homologies be assumed. 

 This is a larger number of lower incisors than is possessed by any known 

 polyprotodont. Further, if the specialized median incisor is not the first 

 but the second as shown by embryology in the macropods (Woodward 

 1 893) , we must assume the ancestral formula to be six. Such an assump- 

 tion is further justified by Woodward's (1. c.) discovery of vestiges of six 

 upper incisors in the macropods and by the occurrence as an abnormality 

 of six upper incisors in Didelphis (Bateson 1894, p. 247; Allen 1901, p. 

 158). If there were six in the upper jaw there may well have been six 

 in the lower, although of course it is not yet known that the casnolestids 

 1 In abnormal cases only. 



