CHARACTERISTICS. 69 



tooth-germs. It has been shown, indeed, that the teeth in 

 the latter part of the series, when first formed, consist of a 

 number of adjacent cusps, and that as development 

 proceeds these cusps become completely separated from 

 each other so as to constitute distinct individual teeth of 

 a simple conical form. 



This discovery is of the very highest import, since it 

 serves to indicate how the numerous simple conical teeth 

 of the dolphins and other toothed whales have probably 

 been derived by the splitting and subdivision of originally 

 complex cusped teeth more or less closely resembling 

 those of the extinct zeuglodonts referred to in the last 

 chapter. 



Being primarily distinguished from the toothed whales 

 oy the total absence of teeth after birth and the presence 

 of whalebone in the adult condition , the whalebone whales 

 present certain other distinctive characters to which we 

 may now briefly allude. 



In the first place, the whales of this group differ 

 externally from all those furnished with teeth in that their 

 nostrils open externally by two distinct longitudinal slit- 

 like apertures ; while, if we cut into the head, we shall find 

 that there is a distinct organ of smell, of which all traces 

 have disappeared among the toothed whales. Moreover, 

 instead of the skull being invariably unsymmetrical in the 

 region of the nose, as it is in the latter group, it retains 

 the normal symmetry ; while, instead of the mere nodules 

 which in the toothed whales represent the nasals of other 

 mammals, in the whalebone whales these bones are fairly 

 well developed. Then again, the lower jaw of any member 

 of the present group may always be distinguished from 

 that of a toothed whale not only by the absence of teeth, 

 but likewise by the circumstance that each of its branches 

 is much bowed outwards in the middle, while their anterior 

 extremities are connected together merely by ligamentous 

 tissue, instead of by a bony symphysis of greater or lesser 

 length. Many other points of difl'erence between the two 

 groups might be cited, but we have especially referred to 

 those mentioned above, for the reason that while the 

 presence of whalebone indicates that in one respect the 



