THE TEETH OF MAMMALS. 285 



(rymmtra. Upper canine-like tooth has two roots ; a single-rooted 



lower pointed tooth closes in front of it. 

 Macroscelis and Petrodromm. The third or outermost incisor is 



two-rooted, long-, and sharp, and plays the part of a canine. 

 Potamoyalc. A small tooth, in no respect different from the other 



premolars, is called a " canine." 



In some of the groups no tooth has been lengthened and 

 pointed, so as to serve as a canine ; in others it is the wrong 

 tooth, i.e., not the same tooth as in the Carnivora, or as in 

 other Insectivora. Consequently, in the Insectivora the 

 elevation of a tooth into caniniform length and character is 

 a mere adaptive modification, which may affect an incisor, 

 or a premolar, or no tooth at all. 



It appears to me that the result of all investigations into 

 the homologies of mammalian teeth may be summed up 

 somewhat in the following manner. 



The evidence of a common pattern, which is traceable in 

 incisors, canines, premolars, and molars (see page 8), would 

 seem to indicate that their special forms have been all 

 derived from modifications of some much more simple form, 

 and that if we are ever to find what might be called a parent 

 mammalian dentition, it will be nearly "homodont:" that 

 is to say, the several teeth will not differ much from one 

 another in size and shape, just as we see to be the case in 

 the dolphin (see fig. 131), or the armadillo. 



It becomes open to question whether the term canine is 

 desirable in dental formula) or in homological determinations ; 

 if we put on one side its functional modifications there is 

 nothing left to distinguish it from other premolars : if we 

 bear in mind its functional development we encounter the 

 anomalies noticed above. 



If we were able to place in unbroken series all the den- 

 titions through which, by progressive modification, the 

 original almost homodont dentition had passed into a 

 highly specialised dentition, like that, say, of the cat, it 

 would be a matter of impossibility to fix upon any point 



