48 CERVICAL VERTEBRA. [CHAI>. 



All the known species of three-toed Sloths (genus Brady- 

 pus] have nine cervical vertebrae, i.e. nine vertebrae in front 

 of the one which bears the first thoracic rib (or first rib con- 

 nected with the sternum, and corresponding in its general 

 relations with the first rib of other Mammals), but the ninth, 

 and sometimes the eighth, bears a pair of short movable 

 ribs. The eighth is perforated by the vertebrarterial canal, 

 but not the ninth. 



The common species of two-toed Sloth (Cholcepus didac- 

 1\his] has seven cervical vertebrae, but a closely allied 

 species (C. hoffmannit] has but six. In both cases the 

 vertebrarterial canal is continued to the end of the series, 

 the sixth and seventh cervical vertebras respectively. 



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 (Phascolomys), 

 Koala (Phascolarctos\ Phalangista, and Kangaroo (Macro- 

 pus], 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 completed by the union of prolongations of the arches 

 inwards. This, however, is not the case with the carnivorous 

 Marsupials. 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- 

 ration. In Perameles and Didelphys the atlas is completely 

 ossified below by a wide intermediate piece, quite as in 

 ordinary Mammals. 



