OF THE AUSTEALIAN MAESUPIALIA. 163 



presence of prototypal conditions in such a central c^roup as the Creodonta indicate 

 that the evolution of the Placentalia is typically a terrestrial one. 



In the Marsupials tlie reverse appears to be the case. As already pointed out by 

 Huxley in 1S80, none of the ineml^ers of this group present an ideal pentadactyl pes, 

 the hallux, whenever present, showing more or less extensive indications of opposability. 

 And, as recently further shown by Dollo (1899), all of the primary modifications of tlie 

 I)es found in the group are in the direction of prehensilism, while all other modifications, 

 wliether indicative of terrestrial or aquatic [Chironectes) adaptation, are derivatives 

 of arboreal phases. Unlike that of the Placentals, therefore, the evolution of the 

 Marsupials is primarily arboreal, and secondarily terrestrial. 



The following summary, representing to a consideral)le extent an adaptation of Dollo's 

 views to the present case, is designed to show the general sequence of arboreal and 

 terrestrial phases in the various marsupial families : — 



a. Primary terrestrial phase Pes adajited for slow terrestrial profiression : 



(Uuideutified Marsupio-Placental stock.) plantigrade, pentadactyl, with short meta- 



podials and more or less radiating digits. 



b. Primary arboreal phases. 



i'. First arboreal phase Pes prehensile for arboreal progression. Hallux 



(Didelphyidae in part.) opposable. 



6^. Second arboreal phase Characters of 6'. 4th digit elongated ; 2ud and 



(Some Didelphyidffi, Phalangeridie except 3rd digits reduced and syndactylous. 

 Tarsi pes -^ 



6\ Third arboreal phase Characters of b"^. Claws of 4th and 5th digits con- 



(Tarsipediuffi.) verted into nails. 



c. Secondary terrestrial phases. 



c\ First terrestrial phase Derivative of b \ Pes elongated ; hallux reduced 



(Dasyuridse in part.) or absent. Plantigrade or subdigitigrade. 



c'. Second terrestrial phase Derivative of b\ Pes variable. Plantigrade or 



(Notoryctidse, Pliascolorayidse, Diprotodou- digitigradc ; pentadactyl, tetradactyl, or func- 



tidse, Peramelidte, Macropodidte.) tionally monodactyl. 



DASYURID^. 



On comparing the distribution of arboreal and terrestrial phases in the Australian 

 Marsupials, we find that they are, as a rule, so completely differentiated as to be 

 distinctive of family divisions. The Dasyuridae, however, present a notable exception, 

 there being evidence of a main line of arboreal or semi-arboreal evolution, with several 

 subsidiary lines of terrestrial adaptation. The general principle of the whole radiation 

 is here illustrated in a single family. 



The main line is represented by the numerous species of Phascogalc, together with 

 the two species D. hallucatiis and ]). niaculatus of Dasi/uriis. Its identification does 

 not depend on the presence of progressive changes, but on the retention of primitive 

 characters in successively larger animals. The subsidiary lines are constituted as 



SECOND SEMES.— ZOOLOGY, VOL. IS. 23 



