NATURE OF TEETH. 59 



that each may have had a totally distinct origin, although 

 the evidence in favour of such a view is, at present at 

 least, far from conclusive. 



In the structure of their teeth, the modern toothed 

 whales differ very widely from the generality of mammals. 

 In the first place, their teeth are always of the simple 

 structure shown in Fig. 21, having conical or compressed 

 crowns, and undivided roots ; while, secondly, there is 

 only one single series developed, the replacement of the 

 anterior ones characterizing the majority of mammals 

 being wanting. From this simple structure of their teeth 

 it has been argued that these cetaceans are among the most 

 primitive of all mammals ; but, altogether apart from the 

 conclusive evidence that all whales (as proved by their 

 breathing air) are derived from land mammals, it has 

 recently been shown by the researches of Dr. Kiikenthal, of 

 Jena, that this view is quite untenable. By examining em- 

 bryos of young cetaceans of this group, that observer has 

 demonstrated that there are actually rudiments of a second 

 series of teeth, which, although never coming to maturity, 

 serve to show that there were once two complete sets, and 

 that the permanent teeth correspond, in part at least, to 

 the milk-teeth of other mammals ; thus indicating that the 

 present state of the cetacean dentition is a degraded one. 

 Hitherto it has not, indeed, been shown by embryology 

 that the teeth of this group were originally of a complex 

 type (although in the case of the whalebone whales this 

 has been demonstrated), but, fortunately, here palaeontology 

 comes to our aid. Thus in the middle of the Tertiary 

 period there occur remains of what may be termed shark- 

 toothed dolphins (squalodonts), in which the permanent 

 teeth are differentiated into distinct series, corresponding 

 to the incisors, canines, premolars, and molars of other 

 mammals ; while, for all we know to the contrary, there 

 may also have been a regular replacement of the more 

 anteriorly placed teeth. In these shark-toothed dolphins 

 the molar teeth, instead of being of the simple structure 

 of those represented in our illustration, were severally 

 implanted in the jaws by two perfectly distinct roots; 

 while their large, laterally compressed, and somewhat fan- 



