374 CETACEA. 



bear on these apparent anomalies, will show that the im- 

 portant variation in form and habits are provided for by 

 the modification of the structures which are essentially 

 the type of the class, rather than by their abolition, and 

 the production of new organs. We will first mention 

 the peculiarities common to the whole order, and then 

 those characteristic of the divisions into which it may be 

 divided. 



The skull of the Cetacea is remarkable for the great 

 development of the jaws, which are extended far in front 

 of the nasal passages, which, consequently, are directed 

 upwards towards the top of the skull, instead of directly 

 forwards as in most mammals. The proportions and 

 relations of the various bones of which the cranium is 

 built up are also much modified, and in most members of 

 the order the whole skull is asymmetrical or distorted, to an 

 extent unknown in any other vertebrate animals. The 

 vertebral column is so formed as to be rigid in front, and 

 flexible behind, for as the Whale has no distinct neck or 

 external division of the head from the trunk, it is obvious 

 that mobility is not here needful ; whereas great flexibility 

 is required further back, to allow of the action of the 

 caudal-fin or tail in swimming and diving. Accordingly 

 we find that the seven cervical vertebrae are very short, 

 and are frequently more or less united by ankylosis, 

 while the succeeding ones are separated by thick cushions 

 of fibro- cartilage, none of them being united to form a 

 sacrum, and the more posterior being but loosely articu- 

 lated with one another. The ribs are, in some groups, 

 very slightly connected with the vertebrae, and the breast- 

 bone or sternum consists either of a single bone, or of 

 several segments. The " flipper " of the Cetacean, 

 although exhibiting externally no divisions or fingers, 

 is supported by the very same arrangement of bones as 



