330 



ANATOMY OF VERTEBRATES. 



added ^ pleurapophysis,' which is ossified from a distinct centre in 

 the Wombat. 



The free or thoracic ribs consist of bony pleurapophyses and 

 gristly hgemapophyses, acquiring bone-earth only in aged Mar- 

 supials : in the Wombat the six anterior pairs articulate directly 

 with the sternum, in the nine following the haemapophyses are 

 attached to one another. The pressure which the trunk of the 

 Wombat must occasionally have to resist in its burrowing work 

 may be the condition of the unusual number of bony arches of the 

 trunk. In the Kangaroo seven anterior pairs of ribs join the 

 sternum ; several of the posterior pairs terminate freely, fig. 211. 



212 



The sternum consists of a longitudinal series of four bones in 

 the Wombat, of five in the Petaurist, and of six in most other 

 Marsupials. The first, or ' manubrium,' is the largest ; in many 

 a longitudinal crest is developed from the middle of its outer 

 surface, which in Wombats and Phalangers is produced, and gives 

 attachment to the clavicles. The first pair of ribs abut upon the 

 anterior angles of the triangular manubrium of the Wombats, but 

 in Dasyures, Opossums, Petaurists, and Kangaroos, the manubrium 

 is compressed and elongated, and the clavicles join the produced 

 anterior end. The haemapophyses articulate at the interspaces 

 of the succeeding sternebers, the last of which supports a broad 

 iiat ' xiphoid ' cartilage. 



The number of vertebrae succeeding the lumbar which are 

 anchylosed together in the sacral region of the spine, amounts in 



