CETACEA. 



37 



of the others are widely open above. In the skeletons of M. 

 americanus, in the same museum, the vertebrae are all free, 

 and the arches, though slender, are complete, and with very 

 slightly developed spinous processes. 



In the Cetacea the seven cervical vertebra usually found 

 in the Mammalia are always present, though often so short 

 and blended together, that it is not easy at first sight to re- 

 cognize their existence. In some genera of both sub-orders 

 all the vertebrae are free, though never allowing of much 

 motion between them ; but more commonly certain of them 

 are firmly united together by bone. Even where the atlas 



Fig. 15. — Section through middle line of united cervical vertebrse of Greenland Right 

 Whale {Bala;na mysticetus), 5. a articular surface for occipital condyle ; e epi- 

 physis on posterior end of body of seventh cervical vertebra ; sfi foramen in arch 

 of atlas for first spinal nerv^e ; i arch of atlas ; 2 3 4 5 6 conjoined arches of the 

 axis and four following vertebrae ; 7 arch of seventh vertebra. 



and axis are separate the odontoid rarely forms a distinct 

 process (it is most distinct in Plafantsta), but still it is devel- 

 oped from an ossific centre of its own, as in other Mammals. 



