42 CERVICAL VERTEBRAL. [chap. 



In the very heterogeneous order Marsupialia (sub-class 

 Didelphia) the cervical vertebrae vary much in their characters, 

 though the number is always seven, as in the great majority 

 of the Mammalia. 



One of the most important variations is in the mode of 

 ossification of the atlas. In the Wombat (P/iasco/omys), 

 Y^.Od\&{Phascolarctos),Phalangista, and Kangaroo (Macropus), 

 there is no distinct ossific nucleus in the inferior arch of the 

 bone, which remains either permanently open in the middle 

 line below, or (as in some of the smaller Kangaroos) is com- 

 pleted by the union of prolongations ot the arches inwards. 

 This, however, is not the case with the carnivorous Mar- 

 supials. In the Thylacine (see Fig. 19) there is a distinct 

 heart-shaped piece of bone in the centre of the inferior arch 

 of the atlas, which appears never to become united to the 

 remainder, as it is still attached by ligament in skeletons 

 otherwise perfectly mature, and is commonly lost in mace- 



Fig. 19 — Inferior surface of atlas of Thylacine ( Tkylacinus cynocefihalus), §. h dis- 

 tinct ossification in centre of inferior arch, with pointed hypapophysial pro- 

 jection. 



ration. In Perameles and Diddphys the atlas is completely 

 ossified below by a wide intermediate piece, quite as in 

 ordinary Mammals. 



As to the other vertebrae, in the Kangaroos the transverse 

 processes are long and slender, and (including the seventh) 

 have a very small perforation close to the base. The inferior 

 lamella arises near the base of the process, and is very 



