254 THE AMERICAN NATURALIST. [Vor. XXXV. 
in the Macropodide (Pls. I and II, Fig. 7). The incipient 
stages are seen in the teeth of the specialized Phalanger, 
Trichosurus vulpecula (Pls. I and II, Fig. Z), and also in those 
of Hypsiprymnodon moschatus (Macropodide). It is of interest 
to note that the lophoid modification is accompanied by a 
hypsodont condition of the crown in the Macropodide, as in 
the perissodactyl Ungulata. 
THE EVOLUTION OF THE FEET IN AUSTRALIAN MARSUPIALS. 
It was pointed out by Huxley in 1880 that none of the 
Australian marsupials possess a normal pentadactyl pes, that 
wherever the hallux is present it presents an extensive move- 
ment of adduction and abduction; in other words, that the 
marsupial foot is typically prehensile. 
Quite recently a careful study of the foot structure of mar- 
supials has been made by Dollo (99), who endeavors to demon- 
strate a derivation of all recent forms from arboreal ancestors. 
Dollo has indicated all the important adaptive modifications in 
Australian marsupials, although he has not discussed their 
phylogenetic significance, so that all that is necessary in the 
present paper is to arrange his types into a phylogenetic series, 
in accordance with his interpretations and the general plan 
noted above for the placentals. 
Dollo has shown that the arboreal marsupials present suc- 
cessive stages of arboreal adaptation, following which the foot 
becomes more and more modified. Thus, beginning with the 
Didelphyidz (Pl. III, Fig. æ), the foot is provided with an 
opposable hallux, and the remaining digits are normal. In the 
Phalangeride (Fig. 5) the hallux is again opposable, but in 
addition the fourth digit is enlarged, and the second and third 
are reduced and syndactylous. Finally, in one member of the 
Phalangeridz, Tarsipes (Fig. c), there is, in addition to all of 
these arboreal Characters, a recession of the claws of the fourth 
and fifth digits. It is thus possible to recognize an arboreal 
line of evolution such as is represented in Pl. III, sc. 
Among the terrestrial or semi 
-terrestrial marsupials, all of 
the forms, 
except the specialized members of the Dasyuridze, 
