No. 442.] 



HABITS IN MAMMALS. 



6 57 



of the head. This is most marked in the hippopotamus but is 

 seen also in the capybara and beaver and other forms. In such 

 forms the eye-sockets may be quite prominent. 



The shortening of the neck is another manifestation of the ten- 

 dency to take on a "fish-like" form, in connection with the ques- 

 tion of locomotion. In all truly aquatic forms this is noticeable. 

 It is brought about by a great shortening up of the cervical 

 vertebrae, and in extreme cases the loss of a vertebra (manatee) or 

 the fusion of some or all of the cervical vertebra; may take place 

 (most Cetacea). The occipital condyles also tend to become 

 flattened out and the odontoid process is reduced. The final 

 result of this process is an almost complete lack of motion 

 between the head and trunk, a condition finding its parallel in 

 the fishes, ichthyosaurs and other truly aquatic forms. This 

 loss of motion in the cervical region is more than compensated 

 for, however, by the greatly increased power of motion attained 

 by the more posterior portion of the body. Here the interverte- 

 bral connections are simplified and the vertebral column rendered 

 more mobile, since for the purpose of swimming, mobility of a 

 certain sort in the posterior part of the body is most useful. 

 The zygapophyses are progressively reduced and lost posteriorly 



pophyses are entirely lost. Also the pleurapophyses or ribs of 

 the sacral region are lost as the pelvis loses its connection with 

 the sacrum (Sirenia and Cetacea). The spinous processes tend 



