WHALES AND PORPOISES 19 



forms of the Toothed ^ Whales [Odontocett). The toothed whales 

 include (besides the extinct Squalodonts) the fresh-water Dolphins, 

 or Platanistidc€ (relatively small forms found in the Amazon and 

 the Ganges, but in a fossil state also in Europe and North 

 America), the True Dolphins (^Delphinida), the Sperm Whales 

 {Physeterida\ and the Ziphioid Whales {Ziphiin^). 



Finally there is the sub-order of the Baleen, or Whalebone 

 Whales {Mystacoceti)^ which differ so markedly from the existing 

 toothed whales that at one time certain zoologists were inclined to 

 believe in the double origin of the whales, that is to say, they 

 would have derived the baleen whales from a different source from 

 that which gave rise to the toothed whales. But this perhaps is 

 due to an exaggeration of the differences between the two great 

 existing groups of the Mystacoceti and Odontocett. In the jaws of 

 the unborn young of most, if not all, whalebone whales there are 

 actually two sets of true teeth, the last of which is, however, 

 absorbed into the substance of the jaw before the birth of the 

 foetus. These two sets of teeth seem to answer to the milk and 

 the mature dentition of other mammalia. In the earliest set of 

 teeth which makes its appearance in the jaws of the foetal whale- 

 bone whale the teeth are much fewer in number than in the 

 succeeding dentition, and they actually offer traces of more than 

 one cusp, thus approximating to the molar teeth of the 

 Zeuglodonts {Jrchaoceti). In this point, as in some others, the 

 whalebone whales offer more archaic features than most of 

 the toothed whales, though in other respects they are a more 

 extreme departure from the ordinary mammalian type. For 

 instance, in the toothed whales the nostrils (except in the foetus) 

 have only a single opening — the blow-hole — whereas in the 

 whalebone whales the orifice is double, as it is in other mammalia. 

 Also among existing whales the Mystacoceti offer slightly more 

 traces of hind limbs than the Odontocett, In the riffht whale, for 

 instance (from which the best whalebone is obtained), there is not 

 only a minute ischium and pelvis but there is a short thigh bone, 



^ As distinguished from the whalebone whales, which have lost all 

 functional teeth. 



