THE SKELETON IN MAMMALIA. VERTEBRAL COLUMN. 443 



forming the rest of the transverse process, and are really of 

 the nature of ribs. The atlas and axis always differ much 

 from the other vertebrae. 



We may pass now to the special characters of the cervical 

 vertebrae in the different groups. In MONOTREMES and MAR- 

 SUPIALS the number of cervical vertebrae is always seven. With 

 the exception of the atlas of Echidna the cervical vertebrae 

 of Monotremes are without zygapophyses. In. Monotremes the 

 transverse processes ossify from centres distinct from that form- 

 ing the body, and remain suturally connected with the rest of 

 the vertebra until the adult condition is reached. The method 

 of the ossification of the atlas in Marsupials varies considerably, 

 thus in some forms such as the Wombats (Phascolomys) there is 

 an unossified gap in the middle of the inferior arch of the atlas, 

 which may remain permanently open ; in Thylacinus this gap 

 is filled up by a distinct heart-shaped piece of bone, while in 

 Didelphys and Perameles the atlas is ossified below in the same 

 way as in other mammals. In Notoryctes the second to sixth 

 cervical vertebrae are ankylosed together. 



The cervical vertebrae of the EDENTATA have some remark- 

 able peculiarities. In the three-fingered Sloth, Bradypus, 

 there are nine cervical vertebrae, all except the last of which 

 have their transverse processes perforated by the vertebrar- 

 terial canals. In a two-fingered sloth, Choloepus hqffmanni, 

 there are only six cervical vertebrae. In the Megatheriidae, 

 Anteaters (Myrmecophagidae), Pangolins (Manidae), and Aard 

 Varks (Orycteropodidae), the cervical vertebrae are normal, 

 but in the Armadillos (Dasypodidae), and still more in the 

 Glyptodonts, several of them are commonly fused together. 

 The fusion affects not only the centra, but also the neural 

 arches, so that the neural canals form a continuous tube. 



In the Glyptodonts there is a complex joint at the base of 

 the neck to allow the partial retraction of the head within the 

 carapace. This arrangement recalls that in Tortoises. 



As a rule the SIRENIA possess seven short cervical vertebrae, 



