No. 412.] THE AUSTRALIAN MARSUPIALIA. 249 
tubercular sectorial pattern, with a main triangle bearing three 
cusps (protoconid, paraconid, metaconid) and an antero-external 
shelf, and a posterior heel bearing one outer (hypoconid) and 
two inner (entoconid and hypoconulid) cusps. 
Foot Structure) — In placental mammals, progressive evolu- 
tion in foot structure, beautifully illustrated in Tertiary Ungu- 
lata, is accompanied by a reduction of certain of the digits, 
and by an elaboration of certain others, the evolution proceeding 
from an ideal pentadactyl condition, such as is seen typically 
in the Eocene Creodonta. Among the Australian marsupials 
the nearest approach to the ideal pentadactyl type is seen 
in certain members of the Dasyuridz (Pl. III, Fig. d). The 
hallux is opposable but slightly reduced, while the remaining 
digits are normal. 
Summing up the above results, we may make the SEEN 
statement that the marsupial stem form possessed the fol- 
dag characters, or more primitive ones: Dental formula, 
1.5, c.l, 9.8, m. 4. Upper molar teeth triangular, with three main 
biis and an outer row of styles; metacone exceeding para- 
cone, its posterior border produced into a trenchant spur ; no 
conules at the base of the protocone. Lower teeth with ante- 
rior triangle bearing three cusps and an antero-external shelf, 
and posterior heel bearing one outer and two inner cusps. Foot 
with hallux completely opposable, the remaining digits normal. 
The fact will now be apparent that the above prototypal 
characters are not found associated in any Australian form. 
All of the Australian marsupials are derived types. On the 
other hand, with one exception they are all found in the 
American Didelphyide. The one exception refers to the pres- 
ence in the latter of four instead of three lower incisors, and 
in this respect the Didelphyidz are more primitive. The molar 
tooth patterns of the stem form are almost exactly reproduced 
in those of the Oligocene Opossums (Ferstheriums (Pls. I and II, 
Fig. a)? | 
! This refers dish to the pes in the case of the marsupials. 
? In the specimen from which these diagrams were taken (Pirineo fugax 
Cope) the outer parts of the upper molars are worn, so that, although there is a 
Conspicuous external ridge, the presence of styles cannot be affirmed. Cf Cope 
(84, Pl. LXII, Fig. 2a). 
