1892.] NATURAL SCIENCES OF PHILADELPHIA. 413 



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 

 npon 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 croAvn, though functionally and in position, 

 it corresponds to the protocone of the molar, and in the finished 

 molariform premolar it occupies the antero-internal angle of the 

 crown. 



The relatively simple form of the fourth superior premolar which 

 has been described, occurs almost universally in the genera of the 

 Puerco Eocene, while the anterior premolars are, for the most part, 

 as yet perfectly simple, consisting of the protocone only. In some 

 genera, however, such as Miockenus, P^ has likewise added a deu- 

 terocone. 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 P^ 

 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, which I 

 have called the tritocone, and which corresponds to the paracone 

 of the molar crown (postero-external cusp), but it cannot be 

 regarded as homologous with that element, because its position with 

 reference to the protocone is entirely different. This stage of devel- 

 opment imitates very closely the trigonodont molar, and very 

 frequently t'liis type of premolar displays the intermediate conules, 

 either anterior, posterior, or both. In position these conules corre- 

 spond to the proto- and raetaconules of the molars, but are obviously 

 not homologous with them. How very gradually this addition of 

 the tritocone may be eflfected is beautifully shown in the series 

 formed by placing together the different varieties and species of 

 Protogonia and Phenacodus. Here the tritocone may be seen in all 

 stages, from a very 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 widely 

 distributed in the middle and upper Eocene, occurring in nearly all 

 perissodactyls and creodonts. AVith some special modifications it 

 persists to the present time in the sectorial of the Carnivora, in 



