VERTEBRATA: MAMMALIA. 685 



bone (the ischium) on each side. A rudimentary femur may 

 be present, and Balcena mysticetus has a cartilaginous tibia as 

 well. There are no clavicles, and the sternum is broad and 

 flat in form. Lastly, the adult is either destitute of teeth, or, 

 with the single exception of the Zeuglodontidcz, 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, except in the Ztuglodonts, 

 they are always of one kind only. 



The skull is often un symmetrically developed, and the 

 maxillae and praemaxillae are greatly prolonged. The nasal 

 bones are short, and the nasal passages are vertically directed ; 

 the epiglottis and laryngeal cartilages being prolonged behind 

 the soft palate in the form of a cylindrical tube, which is 

 practically continuous with the posterior nares, thus allowing 

 the animal to swallow under water without choking. 



The Cetacea may be divided into the five families of the Bala- 

 nidcz or Whalebone Whales, the Delphinidce or Dolphins and 

 Porpoises, the Catodontidcz or Sperm Whales, the Rhynchoceti 

 or Ziphioid Whales, and the Zeuglodontida. Of these the Balce- 

 nidce are often spoken of as the " toothless " Whales, whilst the 

 other four families are called the "toothed" Whales (Odontoceti). 



Fam. i. Halcenidce. The Balanidce. or Toothless Whales 

 are characterised by the total absence of teeth in the adult 

 (fig. 382). Teeth are, however, present in the foetal Whale, 



Fig. 382. Skull of the Right Whale (Balcena mysticettts). (After Owen.) 



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 Greenland Whale), and the Furrowed Whales, in which 



