450 MAMMALIAN ANATOMY 



plicated by the addition of a second cusp upon the inner or lingual 

 side of the protocone, which may be called the deuterocone (Fig. 370, 

 L). This bicuspid tooth represents a pattern from which all the pre- 

 molar types of the higher mammalia may be simply and naturally 

 derived by the continued addition of new parts, which in many groups 

 reach the same or an even greater degree of complication than the 

 true molars. Furthermore, this tooth brings out clearly the important 

 fact that, while in the molar the protocone has shifted to the internal 

 or lingual side of the crown, in the premolar it remains upon the 

 external or buccal side of the crown, just as in the inferior molars. 

 From this it follows that the deuterocone has no exact homologue in 

 the molar crown, though functionally and in position it corresponds 

 to the protocone of the molars and in the finished molariform premolar 

 it occupies the antero-internal angle of the crown (Fig. 370, O). 



11 In the molars the new complications very generally make their 

 first appearance upon the first of the series and then successively upon 



the second and third, and so in the premolars *- - is the first to assume 



new features, and these then advance to the anterior premolars, the 

 first never reaching the full molar pattern, even when the others have 

 exceeded the molars in degree of complexity. 



" The second stage of premolar development consists in the addi- 

 tion of a second external cusp, posterior to the protocone, called 

 the tritocone, and which corresponds to the metacone of the molar 

 crown (postero-external cusp, Fig. 370, E), but it cannot be regarded 

 as homologous with that element, because its position with reference 

 to the protocone is entirely different. This stage of development 

 imitates very closely the trigonodont molar, and very frequently this 

 type of premolar displays the intermediate conules either anterior or 

 posterior or both (Fig. 370, N). In position these conules correspond 

 to the protoconule and metaconule of the molars, but are obviously 

 not homologous with them. How very gradually this addition of the 

 tritocone may be effected is beautifully shown in the series formed by 

 placing together the different varieties and species of Protoyonia and 

 Phenacodus. Here the tritocone may be seen in all stages from a 

 minute and scarcely visible cusp, and gradually enlarging until it 

 reaches the size of the protocone (Phenacodus}. 



" This trigonodont stage of the fourth upper premolar is very 



