CETACEA. 419 



and, as in the Sirenia, there is no sacrum, and the pelvis is 

 only present in a rudimentary form. There are no clavicles, 

 and some of the digits may possess more than three phalanges 

 each. Lastly, the adult is either destitute of teeth or, with ex- 

 ception of the Zeuglodonts, is monophyodont that is to say, 

 possesses but a single set of teeth, which are never replaced by 

 others. When teeth are present, they are usually conical and 

 numerous, and they are almost always of one kind only. 



The Cetacea may be divided into the two sections of the 

 BalcznidcE, comprising only the " Whalebone Whales," in which 

 true teeth are absent ; and the " Toothed Whales " or Odonto- 

 ceti, comprising the living families of the Delphinida (Dolphins 

 and Porpoises), the Catodontida (Sperm Whales), and the Rhyn- 

 choceti (" Ziphioid " Whales), with the extinct family of the 

 Zeuglodontida. 



Fig. 347. Skull of the Right Whale (Balcena ntysticetus) after Owen. 



Fam. i. Balcznida. The Bal<znid<z or Toothless Whales are 

 characterised by the total absence of teeth in the adult (fig. 

 347). Teeth, however, are present in the fcetal Whale, but 

 they never cut the gum. The place of teeth is supplied by a 

 number of plates of whalebone or " baleen " attached to the 

 palate ; hence the name of " Whalebone Whales " often given 

 to this family. They are the largest of living animals, and may 

 be divided into the two sections of the Smooth Whales, in which 

 the skin is smooth, and there is no dorsal fin (as in the Green- 

 land Whale), and the Furrowed Whales, in which the skin is 

 furrowed, and a dorsal fin is present (as in the so-called Finner 

 Whales and Hump-backed Whales). 



The Bal&nidcz are of little geological importance. In Plio- 

 cene deposits have been found remains referred to the Rorquals ; 

 and bones of the Whalebone Whales have also been found 

 in various Post-Tertiary accumulations. It is probable, like- 



