PROFESSOR OWEN ON THE FOSSIL MAMMALS OF AUSTRALIA. 
539 
to the molars is the marked distinction of Phascolomys in the Marsupial series; for, with 
continuous growth, go length of tooth without loss of breadth, depth of implantation 
with, commonly, curvature of socket, and continuation of enamel to the widely open 
base of the tooth. I have no evidence that the first and smallest of the series of five 
grinders in Phascolomys is a premolar or replacing tooth, and view it, therefore, as one 
of the first developed calcified series. It is analogous, in function, in retention, and 
long-continued use, to a premolar of the placental type-dentition. The succeeding four 
grinders in both Phascolomys and Diprotodon are equally members of the first set of 
teeth ; and the last three are homologous with those that are not displaced by vertical 
successors in diphyodont Placentalia. The symbols, therefore, <7 3, <7 4, ml, m 2, m3, 
express, in my opinion, the homologies of the functional molar teeth of Diprotodon with 
those, e.g., so marked in Ilyrax , Hippopotamus, and Sus*. For convenience of com- 
prehension of the teeth symbolized in Plates XXXV .- XL! I. I subjoin woodcuts of 
an instructive phase of dentition in the Hog (fig. 3) and Kangaroo (fig. 4). 
Figs. 3 & 4. 
§ 5. Spinal Column. — Of the atlas there is a portion of the left moiety (Plate XLIII. 
fig. 2) showing the deep articular ca,vity for the occipital condyle of the same side, 
between which and the diapophysis is the outlet of a canal (a) about 3 lines in diameter, 
which traverses the neural arch from within outward behind the upper part of the cavity 
for the condyle. The surface ( z ') for the articular process of the axis is slightly concave ; 
between its upper part and the ridge leading from the hind margin of the neural arch to 
that of the diapophysis there is a deep and wide groove for the passage of the vertebral 
artery into the neural canal. The above-described fragment yields evidence that, as in 
Macropus, Phascolomys, Phascolarctos, and some other Marsupials, the ring of the atlas 
(if indeed it were completed below by bone in Diprotodon) presented only the perforation 
* In my ‘ Anatomy of Vertebrates,’ vol. ii. p. 465, fig. 312 ; vol. iii. p. 346, fig. 276 ; p. 357, fig. 287; p. 377, 
fig. 294. 
