58 TOOTHED WHALES AND THEIR ANCESTRY. 



with the grampuses, porpoises, and dolphins. The most 

 obvious distinction between these two groups, as the terms 

 applied to them indicate, relates to the presence or absence 

 in the adult condition of true teeth. 



Confining our attention in the present chapter to the 

 toothed cetaceans, of which the dolphins and their allies 

 are the least specialized representatives, we find that the 

 two jaws may be in some cases provided with a full series 

 of teeth, while in other forms the number of teeth may be 

 reduced to a single pair, or even, as in the male narwhal 

 (Fig. 9), to a solitary tusk. Whether, however, the teeth 

 be many or few (and in the female narwhal there are 

 none of any functional importance), the structure known 

 as whalebone is never developed in the mouth ; while all 

 the members of the group are further distinguished from 

 the whalebone whales by the circumstance that the 

 nostrils invariably open by a single external aperture, 

 which is very frequently in the form of a transverse 

 crescentic slit, closed by an overhanging valve. In the 

 latter respect these cetaceans are more specialized than 

 are the whalebone whales ; and as the presence of teeth 

 in the former indicates that they could not have been 



Fm. 21. The last six Upper Teetli of the Killer. (After Sir 

 W. H. Flower.) 



derived from the latter, it is evident that the two groups 

 are of extreme antiquity, and have undergone a parallel 

 development. Till recently, it has indeed been considered 

 that they were divergent branches from some common 

 ancestral type ; but, as mentioned in the chapter on 

 " Parallelism in Development," it has been lately suggested 



