MAMMALTA, 579 
The Ce¢acea are usually gregarious and are widely dis- 
tributed marine mammals. They are divided into two sub- 
orders, the Odontoceti and the Mystacocett, which are widely 
apart, 
SUB-ORDER I.—ODONTOCETI. 
The Odontoceti (toothed-whales) comprise the families 
of the sperm-whales (Physeteride), the gangetic dolphins 
(Platanistide) and the dolphins (Delphinide). They have 
_a great number of homodont monophyodont teeth. They 
are more adapted to aquatic habits than the AZpstacoceté in 
one or two respects, such as the entire loss of the olfactory 
organ and the formation of a single external nas. 
The Physeteride are large predaceous marine forms, 
such as the sperm-whale. The Platandstide comprise the 
estuarine or freshwater river-dolphins, such as the blind- 
dolphin of the Ganges, The large family of the Dehinide 
includes the dolphins and porpoises of European seas, the 
narwhal (AZonodon) of Arctic seas, with a single twisted tusk 
formed of a left upper incisor, and the “ killers” (Orca). 
SUB-ORDER II.—MYSTACOCETI. 
The Afystacoceti (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 vertebree only, and only one pair meet the small 
sternum, features which give the baleen-whales a greater 
freedom of respiration than the Odontocet. 
On the other hand, the external nares are paired and 
partially covered by the nasal bones and there is a distinct 
