116 VERTEBRATA ! MAMMALIA. 



covered with a smooth skin, beneath which is a layer of 

 fat called blubber. Their brain is broad, neck very short, 

 and the second cervical vertebra is without a tooth-like 

 process. 



The breathing orifice of the Cetaceans, which corre- 

 sponds to the nostrils of ordinary mammals, is situated in 

 the top of the head, and is popularly called the " blow- 

 hole " ; through this the water which has been taken into 

 the mouth is sometimes " spouted " to a great height. 



There is a group of extinct cetaceans which are called 

 ZEUGLODONTS. Their remains abound in the Tertiary of 

 Alabama, and show that these animals attained the length 

 of eighty feet or more. The name is derived from the 

 Greek zeuglon, a yoke, and odous, a tooth. They have 

 teeth which are called yoke-shaped. 



The DENTICETES or Toothed Whales as the Dolphins, 

 Sperm "Whales, etc. have true teeth, and no whale-bone 

 or baleen. The Dolphins or Delphimdse, of which there 

 are many genera, such as Dolphins proper (Fig. 123), 

 Porpoises, White Whale (Beluga), Narwhal (Monodori), 

 etc., are from eight to twenty feet in length. The Sperm 

 Whales, or Physeteridse attain the length of sixty or 

 seventy feet, and the head (Fig. 125), constitutes about 

 one third of the whole animal. The well-known sperm 

 oil and spermaceti are obtained from the last-named 

 cetaceans. The Sperm Whales belong to the warm re- 



gions. 



The MYSTICETES or Whale-bone Whales have the upper 

 jaw provided with baleen or whale-bone (Fig. 126), and 

 they have true teeth only before they are born ! To this 

 group belong the Finback Whales, or Balsenopteridse, which 

 in most cases have a dorsal fin more or less developed, and 

 which have short baleen. Some of the Finbacks attain 



