198 EVOLUTION OF MAMMALIAN MOLAR TEETH 



attributed to the molars, even when the final results are the same. It 

 is another remarkable instance of convergence in evolution, and proves 

 that similarity in form and in position affords an unsafe guide to serial 

 homology (see, however, Chapter XL). 



INFERIOR PREMOLARS. 



As shown in the foregoing comparison of the Jurassic mammals, 

 and in other primitive mammals, the primitive form of the lower 

 premolar is a simple, more or less recurved cone, obviously corresponding 

 to the protocone of the molars, and implanted by a single fang. 



The metamorphosis of the inferior premolars also begins with the 

 fourth premolar, and extends anteriorly. While the order of cusp 

 development is less regular and constant than in the upper teeth, we 

 find that the lower premolars may be more closely compared with the 

 lower molars, that is, there is more evidence of serial homology between 

 the cusps of the lower premolars and lower molars. This fact fortunately 

 enables us to use the same terminology for the premolars as for the 

 molars. Professor Scott was led to take a different view, and proposed 

 several new homologues in the lower premolars, such as the paraconid, 

 deuteroconid, metaconid, tetartoconid, which may be eliminated by a 

 somewhat different interpretation of the development. 



Comparison of the lower premolars should be made, not with the 

 molars of the Triconodonta, but with the molars of Trituberculates. 

 Such comparison shows that the premolar evolution may be interpreted 

 as substantially similar to the molar evolution. 



The initial stage is the simple, single-fanged, conical cusp, the 

 protoconid of Dromatherium (Figs. 3, 195 A}. 



Even in the Jurassic mammals the second stage appears in the two- 

 fanged crown with more or less developed posterior basal cusp, which 

 corresponds in position and function with the talonid or hypoconid 

 of the tritubercular molars of such a type as Amphithcrium (Fig. 15). 

 In the subsequent evolution of the crown this hypoconid remains on 

 the outer or buccal side of the crown as in the molars ; it is therefore 

 practically homologous serially with the hypoconid of the tritubercular 

 molars rather than with the metaconid of the triconodont molars. 

 Most of the existing Unguiculates as well as some recent, and many 

 extinct Ungulates, retain more or fewer premolar teeth which depart 

 but little from this type. 



As a third *t</<- an anterior basal cusp comparable to the paraconid 

 both of the tritubercular and triconodont molars is added. This cusp, 

 in fact, corresponds in the subsequent evolution of the teeth with the 



