146 MA.RSrPIALIA. 



Subclass METATHEEIA. 

 Order MARSUPIALIA. 



The homology of the cheek-teeth of the Marsupialia having been 

 recently worked out by 0. Thomas \ it will suffice to mention that 

 the tooth which is either preceded by a milk-molar, or which is 

 homologous with the one so preceded, corresponds with the fourth 

 premolar of the Eutheria ; while the four teeth which usually 

 occur posteriorly to the latter are reckoned as true molars, and 

 numbered consecutively. The tooth immediately in advance of the 

 fourth premolar corresponds to the third premolar of the Eutherian 

 series ; but in the numerous existing members of the Polyprotodont 

 suborder in which there are three premolars, the first of these corre- 

 sponds with the first (and not with the second) of the Eutherian 

 series ; judging, however, from Plagiaulaw, it is probable that the 

 same rule may not hold good for all those few members of the Dipro- 

 todont suborder in which three premolars are present. A peculiarity 

 in regard to the serial homology of the two premolars, which 

 occur in one genus of the Polyprotodont family Dasyuridce, will be 

 noticed under that head. 



Suborder MARSUPIALIA DIPROTODONTIA. 



Incisors ^^ \ The central pair of upper incisors and the one 

 pair of lower incisors large and cutting. Canines absent or small. 

 Molars with bluntly tuberculate, or transversely or longitudinally 

 ridged crowns. There are usually but two premolars in existing 

 forms, but four of these teeth are present in one species of the 

 Mesozoic Plagiaulax. The fourth premolar is frequently of a simply 

 secant structure, and may be considerably longer than the first true 

 molar. 



Family PHASCOLOMYID^. 



Dentition^ :— I. |, C. ^, Pm. \, M. \. All the teeth have persistent 

 pulps. The incisors are large and scalpriform, with enamel only on 

 the anterior surface. The cheek-teeth (woodcut, fig. 26) are curved, 

 the convexity being inwards in the upper and outwards in the lower 

 jaw ; the premolar, which has no milk predecessor, is usually single- 



^ Abstract in Proc. E. Soc vol. xlii. p. 310 (1887). 



'^ It may possibly prove that there vfQVG more than a single pair of upper 

 incisors in Phascolonus. 



