OF THE AUSTEALIAN MARSUPIA.LTA. 191 



Of the Pcramelidsc the deciduous premolars are represented in specimens of three 

 species of Feramelcs {P. obesula, P. nasiUa, and P. Cockerelli), as well as in Choeropns 

 and Thylacomys. In all of these, as in the Dasyuridse, they are greatly reduced as com- 

 pared with those of the Didelphyid;ie. The specimen of P. nasuta presents the same 

 appearance as one figured by Wilson and Hill (1897, pi. 32. fig. 78), the minute deciduous 

 tooth heing supported on the posterior slope of its enlarged successor. A reduced 

 condition of the deciduous teetli has been figured by Flower (1869) for P. Bougaliivlllel 

 and by Hose (1892) for P. Lorcyana. 



None of the specimens representing the smaller species of the Phalangeridse show the 

 deciduous j^remolars. In Phascolarctus, ns poiuted out by Thomas (1887 h) and 

 Dependorf (1898), they are wholly vestigial. In the final members of the Phalangerinse, 

 Phalanger and TricJiosnrus, as well as in the Macropodidfe, we find a modification of 

 quite the reverse kind, the deciduous teeth being well developed and functional until 

 well on in life. In the phalangeriue forms they perform the same sectorial functions 

 for the young as the posterior premolars do for the adult, while in the Macropodidse, 

 where the median premolars are retained as sectorials in the young, the deciduous teeth 

 take on, or perhaps retain, a molariform shape. In the Phascolomyidae the deciduous 

 teeth have aj^parently been reduced in the ordinary way. In view of the concise 

 evidence in the DasyuridfB and Peramelidfe of retrogression from a didelphyid type, it 

 is surpi'ising to find the deciduous teeth so well developed in the most advanced forms 

 of Marsupials, such as the advanced Plialangerinse and Macropodidae, where one mio-lit 

 expect to find them wholly absent. Whatever may have been the original cause of the 

 reduction of one dentition — Leche has ascribed it to the peculiar suckling conditions 

 attendant on premature birth— it appears probable that in these forms the reduction of 

 the last element, namely, the deciduous premolar, has been checked by its becoming 

 serviceable in the secondary herbivorous evolution. 



Foot-structure. — With regard to the foot-structure of the Australian forms it will be 

 seen that the prototypal characters are distributed over two families. The Dasyuridjc 

 are primitive in the non-syndactylous and unreduced condition of the second and third 

 digits. The Phalangeridae are primitive in the possession of a full complement of well- 

 developed and transversely striated plantar pads, and of a fully developed and completely 

 opposable hallux associated with a non-elongated condition of the whole pes. It is 

 apparent that the two types respectively characteristic of tliese two families are deri- 

 vatives of a common form combining the prototypal characters of both. This common 

 form is exactly represented by Ilarmosa (PI. 7. fig. 7) among the Didelphyidie, with the 

 partial exception that there is here, in some species, an indication of the syndactylous 

 condition of the Phalangeridie. 3larmosa is completely arboreal, just as tlie ancestor of 

 the Australian forms must have been. The conditions in this genus are repeated in 

 Caluromys. Peramys, the genus which is most primitive in dentition, is slightly 

 specialized in a terrestrial direction in foot-structure (PI. 7. fig. 1), but, except for tlie 

 partial reduction of the plantar pads and a tendency to shorten the fifth digit, retains 

 the character of a prototypal form. It is an interesting fact that we have here in 



