516 



todon. A similar difference is shown by the two species of Wombat of which the atlas 

 is figured in Plate XCVIII. 



The bony arch overroofing the neural canal above the articular cups (z, z, Plate 

 ( XXVI. fig. 1) is lower and flatter than in Phascolomys, and more resembles that part 

 of the atlas in Macropus. But the upper surface of the arch, instead of being flattened 

 as in Macropus, rises into a large median tuberosity (fig. 1), and more resembles the 

 shape of that part in Phascolomys. In the minor extent of the neurapophyses below 

 the articular cups both Diprotodon and Nototherium more resemble Phascolomys 

 platyrhinus (Plate XCVIII. figs. 3 & 4) than they do Phase, latifrons or Macropus 

 rufus (Plate LXVII. figs. 2 & 3, np). 



The inward extension of the neurapopbysis from the lower half of the articular cup, 

 indicative of the level of the floor of the neural canal (ib. n), gives a square form to 

 the vacuity, the lower half of which was mainly filled by the centrum of the atlas 

 ("odontoid process"). The anteroinferior angles of the neurapophyses are produced 

 into low conical processes (ib. mp). The lower opening of this vacuity was doubtless 

 closed by ligamentous matter at h. 



The inner aperture of the canal, answering to c i, figs. 1 & 3, Plate XCVIII., in 

 Phascolomys, is present at the side of the neural canal, near the upper part of the 

 articular cup (z). Its course is transversely outward, the length of the canal being 

 1 inch 3 lines ; its diameter is 4^ lines in Diprotodon. It is continued by an open 

 groove, gradually widening, to the interspace between the prezygapophysis (ib. z) and 

 the diapophysis (ib. d). The diapophyses, at their origin, have greater anteroposterior 

 than vertical thickness, but swell out soon after their origin into rough irregular sub- 

 spherical tuberosities. 



The under part of the neck of the process is grooved by the vertebral artery. A 

 process from the lower and outer side of the neurapophysis tends to circumscribe the 

 groove below, but terminates in a point, leading to an open channel, as in Macropus 

 (Plate LXVII. figs. 2 & 3, v) and in Phascolomys platyrhinus (Plate XCVIII. figs. 3 & 

 4, v). The postzygapophyses (Plate CXXVI. fig. 2, z) are subtriangular, feebly con- 

 cave, with the margin sharp and prominent, save along the median part, which subsides 

 upon the vertical wall bounding the vacuity for the centrum. 



Subjoined are admeasurements of the atlas in the two species: — 





Diprotodon. 



Nototherium. 





in. 



lines. 



in. 



lines. 



Extreme length or antero-posterior diameter . . 



5 







3 







Extreme breadth or transverse diameter 



12 







9 









5 



9 



4 



6 



Extreme length of neural canal between fore and hind 













1 



6 



1 







Extreme breadth of neural canal above hind articular 













3 



2 



2 



10 



