OF THE AUSTEALIAiV MAESUPIALIA. 



89 



opinions expressed by Thomas (1887), Leche (1892, 1893, 1895), Woodward (1896), 

 Wilson and Hill (1897), Dependovf (1898), and Lydekker (1899) leads to no definite 

 conclusion ; and it therefore appears inadvisable at the present time to adopt any system 

 of nomenclature which will imply, intentionally or otherwise, definite homologies with 

 the Placentals. Thomas (1895) has recently adopted the plan of designating the three 

 premolariform teeth of Marsvipials, in the order of their position from before backwards, 

 as anterior, middle, and posterior; and this plan has been adopted throughout the 

 present paper (with the substitution of the term median for middle), as presenting the 

 advantage over numerical systems that it conveys the idea of homologies only within 

 the limits of the Marsupial group. It may be observed that even these terms have 

 some disadvantages, since, in order to express homologies, it has been necessary in 

 several cases to disregard their descriptive applicability. In the advanced members of 

 the Dasyurinse, for example, where the posterior premolars have disappeared, the 

 remaining teeth are still distinguished by the terms anterior and median. 



Fig. 1. 



Cusp-nomenclature of upper and lower molars. 

 A, R, crowu aud profile of upper, C, I), of lower molar of Peiatherium ; E, F, upper and lower molars of Trichosunis ; 

 G, lower molar of Pemmeles DowjainviUii ; H, upper molar of Pseudochirus ; I, J, upper and lower molars of 

 Macropus. Abbreviations : pr., protocoue ; pit., paracone ; mi?., metacone ; hy., hypocone ; pL, protocouule ; 

 Hi/., metacouule; a, h, \, c,, f, c,,, external styles; pr''., protoconid ; pa''., paraconid ; j/it"'., metaconid ; 

 hy''., hypoconid ; en''., entocouid ; hi''., hypocoiiulid ; a.e.s., autero-external shelf. 



The molariform teeth of the adult animals have been designated throughout as first, 

 second, third, and fourth molars, the first tooth being regarded as a true molar rather 

 than as a formerly deciduous premolar whose successor has been aborted (Lydekker, 

 1899). 



The nomenclature used in describing the patterns of the molars is that proposed by 

 Osborn (1891) in connection with the tritubercular theory of molar evolution. An 

 exception has, however, been made in the case of the external cusps or " styles " in the 

 upper teeth of polyprotodont forms. Certain of these elements, as they occur in the 



