OF THE AUSTRALIAN MARSUPIALIA. Ill 



to act as stops for the louver teeth. The lower incisors appear to be already modified 

 throughout as cutting rather than simply piercing organs. 



The incisor modifications of FliascolarctHs indicate a much more specialized condition 

 than those of Pseitdochirus. The chief ditference relates to the tendency towards the 

 development of rodent characters. The lower teeth meet the upper at a wide angle 

 (about 90"), so that instead of cutting by their edges against the tips of the upper lateral 

 teeth they cut entirely by their tips against those of the median upper ones. The upper 

 lateral teeth therefore act as stops. The median upper teeth are rounded in section and 

 flattened from before backwards at the tips. The lateral teeth are slender, and at most 

 only slightly thickened distally as in most species of Fseudochtrus. Tlic lower teeth arc 

 lanceolate Allien unworn, and scalpriform when worn. In the peculiar manner in which 

 the median upper and lower teeth are fitted together, as also in the slenderness of the 

 lateral teeth, Phascolarctus shows an interesting apj)roximation to JDqirotodon, although 

 there is no tendency in the former towards the differentiation of enamel bands. 



As in the case of the molars, there is nothing primitive in the characters presented by 

 Phascolarctus as compared Avith those of the PseudocJiinis type. The unworn lower 

 teeth of Fhascolarctus, on the other hand, furnish direct evidence that a cutting 

 modification like that of the latter type is the original condition from which the 

 scalpriform one has been derived. 



Upper Canines. — These teeth show few points of phylogenetic interest. In all 

 members of the series they are reduced and apparently f unctionless, this condition being 

 a specialized one, as in the advanced Phalangerinpe, and contrasting witli the primitive 

 one in the smaller members of the latter group. They are short, often laterally 

 compressed, and in general bear little resemblance to normal canines. 



Functional Fremolars. — Fsemlochirus and Fetauroides are primitive in respect to the 

 possession of a full upper premolar formula. The anterior teeth are reduced, the 

 median moderately developed, and the p)osterior slightly enlarged, the condition being 

 also fairly primitive. The median teeth sliow slight indications of compression of tlu' 

 edges, and this character is still more pronounced in the posterior teeth. The latter, 

 unlike the compressed teeth of the advanced Phalangerimc, have their original tijjs in 

 the centre instead of at the anterior end. These teeth show signs of the formation of 

 an internal ledge and of a postero-intcrnal cusp. In the lower jaw the conditions 

 are more advanced, the only functional teeth being the posterior. Like the upjior 

 teeth theu- edges are compressed, and they also show indications of an anterior accessory 

 cusp. 



The condition in Fhascolarctus appears to be a direct advance on this type. The 

 anterior and median upper premolars have disappeared, while the posterior have become 

 enlarged. The latter are of much the same character as in Fseudochirus, but the 

 edge is more uniformly trenchant and the internal ledge better developed. The lower 



20* 



