132 FIELD MUSEUM OF NATURAL HISTORY ZodLOGY, VOL. XIV. 



some polyprotodonts. In Trichosurus, for example, and in C&nolestes, 

 the median upper incisors are less differentiated than in some species of 

 Phascologale. 



For the development of diprotodonty from such a form as Marmosa 

 the following are among the changes which would be necessary: Reduc- 

 tion in number of incisors; enlargement of median lower incisors; reduc- 

 tion in size of canines and anterior premolars; decrease in the depth of 

 the mandible; increase in craniofacial length; formation of short diastem- 

 mata between at least some of the teeth ; assumption of proclivous and 

 fully terminal position by median lower incisors; assumption of wholly 

 lateral position by posterior incisors; lateral compression of incisors and 

 development of long and f orwardly directed cutting edges, especially in 

 those of the upper jaw. All of these modifications are found to a con- 

 siderable degree in Myrmecobius and in these respects this form re- 

 sembles the diprotodonts more closely than do any of the didelphids or 

 dasyurids. Whether these characteristics were more or less pronounced 

 before the present degeneration of teeth began cannot be said, but at 

 least it must be conceded that Myrmecobius has some claims to a position 

 in or near the line of the diprotodonts whether it be regarded as an 

 ancient type derived from Jurassic forms or a more recent one proceeding 

 from the dasyurids. 



Leaving Myrmecobius for further discussion later, it may now be 

 inquired whether the peramelids could have furnished the beginnings 

 of diprotodonty. Although Bensley finds in the Peramelidae many 

 indications of an ancestral relation to the modern diprotodonts and an 

 advance over the primitive dasyurids and didelphids, he is not inclined 

 to regard the diprotodont dentition as directly derived from them. In 

 this he was influenced largely by the undifferentiated condition of the 

 median upper incisors in the peramelids; by the retrogressive (?) char- 

 acter of the upper canines and premolars, the existence of primitive and 

 advanced stages of the hypocone in both groups, and the hypsodonty 

 of the peramelids. The development of the hypocone and the hypsodont 

 modifications in the peramelids are progressions beyond the most 

 primitive stages of the known diprotodonts. But they need not be 

 considered wholly homoplastic and their inception may have preceded 

 the divergence of the peramelid stem. In any case they do not preclude 

 a belief in a common ancestry for the peramelids and the diprotodonts 

 and one which was not very remote. Such an ancestry at most need only 

 be removed to a stage in which the hypocone and the hypsodont^ were 

 approximately at the most primitive stage common to both peramelids 

 and diprotodonts. This would carry us back to a brachyodont condition 

 but not to a total absence of the hypocone. So of the two characters 



