478 EXTINCT AND VANISHING MAMMALS 



(2) Iniidae, the fresh- water dolphins of the Amazon, the 

 Plata, and the Yangtse Rivers. These are similar to the Ganges 

 form, except for some important details of structure. The 

 Yangtse species is discussed in this section. 



(3) Delphinapteridae, the white whale, or beluga, and the 

 narwhal. 



(4) Delphinidae, the dolphins and porpoises. This is a large 

 group, containing many diverse genera. Some of the species 

 are rare, others are abundant. The false killer whale is con- 

 sidered here because for some time it was thought to be extinct. 



(5) Ziphiidae, beaked whales. The beaked whales have only 

 one or two teeth on either side of the lower jaw, and both jaws 

 form a beaklike structure. The ten species of the genus 

 Mesoplodon, all poorly represented in museum collections, are 

 thought to be vanishing mammals. 



(6) Physeteridae, sperm whales. There are two genera in 

 this family, one a pygmy type, the other reaching 60 feet in 

 length. The sperm whale, or cachalot, was much sought by 

 whalers during the eighteenth and nineteenth centuries and 

 was thereby greatly reduced in numbers; it is now thought to 

 be on the increase. 



Mystacoceti, the whale-bone whales. These have no teeth 

 after birth; instead they have fringed plates of baleen which 

 act as sieves to retain the small animals on which the true 

 whales feed. Three families are recognized: 



(1) Rhachianectidae, the gray whale of the northern Pacific 

 Ocean. The unique species combines characters of the fin 

 whales and the right whales. It is seriously threatened by the 

 Japanese whale-fishery. 



(2) Balaenidae, right whales. The right whales have neither 

 dorsal fin nor throat-folds. Three species of as many genera 

 are discussed here. 



(3) Balaenopteridae, fin whales. These have a dorsal fin 

 and the throat is plicated. Five species of three genera are 

 seriously affected by modern whaling operations and will be 

 exterminated unless protective measures are made effective. 

 J. E. H. 



