SKELETON OF THE WALRUS. 



159 



thrown during retraction of the head, exceeds that in most 

 other mammals, and ahiiost reminds one of the extent of 

 flexion of this part of the spine in birds. 



Fig. 27. 



CT^ 



SKELETON OF THE WALRUS (Trichccus rosmanis). 



In the wah'us, the skeleton of which is here selected to 

 exemplify the phocal modification of the mammalian skele- 

 ton, the vertebral formula is : 7 cervical, C, 11 dorsal, D, 

 5 lumbar, L, 3 sacral, S, and 9 caudal, cd. As, in conse- 

 quence of the presence of hind-limbs, a sacrum is now 

 established, the characters of the above five kinds of 

 body-vertebrae, as defined in man and other mammals, 

 may here be given : the cervical or neck- vertebrae " have 

 perforated transverse processes," the dorsal vertebras 

 "bear ribs;" the lumbar vertebras "have imperforate 

 transverse processes and no ribs;" the sacral vertebra3 

 " are anchylosed together ;" the rest are caudal vertebrae, 

 whatever their modifications. In the above characters, 

 the term "rib" is given to the vertebral element called 

 "pleurapophysis," when this is long and movable; that 



