ii2 FIELD MUSEUM or NATURAL HISTORY ZOOLOGY, VOL. XIV. 



NUMBER AND HOMOLOGIES OF TEETH. 



The teeth are 46 to 48 in number. In the upper jaw there are one 

 pair of terminal incisors and three pairs of lateral ones, a pair of sharp, 

 well-differentiated canines, three pairs of more or less triconodont pre- 

 molars, and four pairs of molars, the first and second of which are 

 quadrate and the third and fourth subtriangular. In the lower jaw there 

 is a pair of long terminal chisel-like incisors resembling those of the 

 most pronounced diprotodont dentitions. Behind these in lateral 

 position are four and sometimes five pairs of small unicuspids, practically 

 undifferentiated from each other, which are interpreted as two (or three) 

 incisors, one canine, and one premolar. Next are two pairs of double- 

 rooted premolars and four pairs of molars. The dentition, therefore, is 

 a rather highly modified one but retains approximate numerical agree- 

 ment (at least in certain specimens) with the polyprotodont dentition 

 usually regarded as generalized. With the exception of the didelphids, 

 which have five upper incisors, and Myrmecobius, which has super- 

 numerary molars, no living polyprotodont has more teeth than C&no- 

 lestes. The formula may be written: 



I. ~ C.j;Pm.|;M. -^5x2=46-48. 



This is essentially the classification made by Thomas (1895) who 

 divided the four unicuspids behind the median incisor into two incisors, 

 one canine, and one premolar. "Any other determination, " he says, 

 "would involve the presence of four incisors or four premolars, each 

 equally unlikely." It now appears that, although the normal number of 

 lower incisors is three as decided by Thomas, the presence of four would 

 by no means have been unlikely. The number of lower unicuspids or 

 intermediate teeth is normally four, but a variation in which there are 

 five is not infrequent. In one specimen (Field Museum No. 18603), 

 there are five on the right side of the jaw and four on the left. Bensley 

 (1903, PI. 5, fig. 38) has figured a specimen belonging to the British 

 Museum in which there are five on each side of the jaw. Others, so far 

 as examined, have four unicuspids on each side, but since the total 

 number of specimens is very small the percentage of variation is high. 

 It is. possible, therefore, that these aberrant specimens represent a 

 condition once normal and in which the lower antemolar formula was 

 equal to that of the didelphids and in excess of other living marsupials. 

 This is further suggested by the occurrence of the same lower antemolar 

 formula in the extinct genus Halmarhiphus, regarded by Sinclair as the 

 direct ancestor of C&nolestes. A closely allied form, Garzonia, has as 



