209 



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, d3, <Z4, ml, m e 2, mS, 

 express, in my opinion, the homologies of the functional molar teeth of Diprotodon with 

 those, e.g., so marked in Hyrax, Hippopotamus, and Sus*. For convenience of com- 

 prehension of the teeth symbolized in Plates XIX.-XXVII. 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 XXVIII. 

 fig. 2) showing the deep articular cavity 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 1 ) 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. 



