140 DE. B. A. BENSLEY OX THE EVOLUTIOIS" 



type involved an incipient hypocone and slightly reduced paraconid and liypoconulid. 

 The omnivorous stage in the evolution of the Phascolarctinge must have been of 

 extremely short duration. 



Incisors. — Within the limits of the genus Pseudochirus we find several variations in 

 the characters of the incisors, especially those of the upper jaw. There is, however, 

 evidence of a gi'ouud type corresponding in a general way to that represented by the 

 smaller Phalangeringe. 



In P. Porhesi, a small form showing, for the most part, primitive conditions, the 

 median upper incisors are of an almost insectivorous type. Their tips are rounded in 

 section and pointed, and they project a considerable distance beyond those of the lateral 

 teeth. They are directed towards one another in the middle line. The second and third 

 upper teeth are suhequal in size, the second shows a very slight thickening of the tips. 

 The enlarged lower teetli are lanceolate and only slightly curved ; their tips, which are 

 rounded, work against tlie posterior sides of the median vipper teeth, while their sides 

 cut against the lateral ones, especially the second. 



In P. Alhertisi the median upper incisors are much as in P. Forhed, but are stoater. 

 The second iipper teeth have bulb-like tips. The third upper teeth are relatively small 

 and unmodified. The lower teeth are flattened like those of P. Forbesi, but, unlike the 

 latter, their external cutting-edge is concave, so as to fit the convexity of the upper lateral 

 teeth. Somewhat similar conditions are found in P. Cormnce, P. cupreus, and P. Bahli. 

 In P. CorinnfB, however, the median upper incisors are very much elongated, projecting 

 downwards, but only slightly forwards, and are also separated from one another in the 

 middle line, this development producing an appearance similar to that seen in certain of 

 the Insectivora and Ilyracoidea. Much the same modification is found in P. Pahli, 

 althoush the teeth are not elonorated to the same extent as in P. Corlnnce. With 

 reference to the second upper teeth, it is difiicult to decide whether or not their tips are 

 bulbous in P. Corinnce and P. Bahli, these teeth being much worn in all of the specimens 

 examined. In P. Dalili the external cutting-edges of the lower incisors are not concave 

 as in the remaining species except P. Forbesi. 



In P. 2)ereffriinis, P. occidentalis, P. Cooki, and in Petauroides volans we find modifi- 

 cations of a slightly diflferent kind. The median upper incisors are reduced, so that they 

 project only slightly beyond the lateral teeth. They present a compressed appearance 

 which is due to the excavation of their postero-internal surfaces where they come in 

 contact with the lower teeth. They are separated at their bases and approximated at 

 their tips. Apart from their more reduced characters, they resemble somewhat those of 

 Petaurus. The second and third teeth are much as in P. Forbesi, being subequal, 

 slender, and on the whole unmodified. The lower teeth are only slightly curved and are 

 fully lanceolate. The internal edges are not concave. 



The above facts, although not exhaustive, are sufficient to indicate that the main 

 feature on which the variations develop in this genus is one in which the median upper 

 incisors are pointed, elongated, and procumbent, the lateral incisors subequal and inturned 



