MAMMALIA. - 579 



The Cetacea are usually gregarious and are widely dis- 

 tributed marine mammals. They are divided into two sub- 

 orders, the Odontoceti and the Mystacoceti^ which are widely 

 apart. 



SUB-ORDER I. — ODONTOCETI. 



The Odontoceti (toothed-whales) comprise the families 

 of the sperm-whales (Physeteridce), the gangetic dolphins 

 ( Platanistidce) and the dolphins (Delphinidce), They have 

 a great number of homodont monophyodont teeth. They 

 are more adapted to aquatic habits than the Mystacoceti in 

 one or two respects, such as the entire loss of the olfactory 

 organ and the formation of a single external nas. 



The Physeteridce are large predaceous marine forms, 

 such as the sperm-whale. The Platanistidce comprise the 

 estuarine or freshwater river-dolphins, such as the blind- 

 dolphin of the Ganges. The large family of the Delphi?iidcB 

 includes the dolphins and porpoises of European seas, the 

 narwhal {Monodon) of Arctic seas, with a single twisted tusk 

 formed of a left upper incisor, and the "killers" (JDrca). 



SUB-ORDER II. —MYSTACOCETI. 



The Mystacoceti (baleen-whales) have teeth only in the 

 embryonic young, which never become functional. They 

 are replaced by a row of baleen-plates suspended from the 

 upper-jaws, forming the so-called "whalebone." Their 

 edges are frayed and they act as a sieve for separation of 

 the food from the water. The head, especially the facial 

 portion, is enormously developed, and the rami of the lower 

 jaw are only connected by ligament. The whales feed upon 

 small pelagic organisms, such as pteropods and . certain 

 Crustacea. The buccal cavity is huge and becomes filled with 

 sea-water containing such pelagic organisms. The former is 

 then driven out between the baleen-plates by elevation of 

 the tongue, the latter being retained and swallowed. In 

 Mystacoceti the ribs are attached to the transverse processes 

 of the vertebrae only, and only one pair meet the small 

 sternum, features which give the baleen-whales a greater 

 freedom of respiration than the Odontoceti. 



On the other hand, the external nares are paired and 

 partially covered by the nasal bones and there is a distinct 



