The morphology of the male phallus is described for 19 of the 23 genera of New Guinea rodents; 28 species are included among the 72 penises examined. A total of 66 characters was studied on each penis and the resulting data were carefully analysed for phylogenetic evidence. An interpretation of the relationships among the taxa examined, based solely on the phallic data, is presented in a phylogenetic tree. Comparisons are then made with the earlier phylogenetic hypotheses, which were based on dental and external body features only. Both approaches indicate four major subdivisions in the endemic fauna.
An hypothesis is proposed that accounts for the evolution of all endemic forms from only two invading stocks, one in the Pleistocene giving rise to endemic species of Rattus, and one in the Miocene leading to all the other native rodents. In addition, four widespread modern species have reached New Guinea with the help of man. The extant genus most like the hypothetical original murid immigrant is Anisomys. Evidence is presented indicating that Hyomys should be placed in the XJromys group rather than with Pogonomys and Mallomys. Finally, Pogonomelomys is shown to be almost certainly polyphyletic.