EVOLUTION OF THE PREMOLARS 



197 



passed through by the fourth premolar, until it reaches the size of 

 a protocone (Figs. 197, 198). 



pr tr 



FIG. 198. Complication of the superior premolars in Coudylarthra. 1. PS, p4 of Phenaeodus 

 prinuevus, showing a complicated j>4. 2. P3, pi of Phenaeodus ioortmn,ii with simpler p*. 3. P of 

 ilri'ta. After Scott. 



This stage is very widely exhibited in the Middle and Upper Eocene 

 Ungulates and Creodonts and persists till the present time, with some 

 modifications in the sectorial or carnassial fourth premolar of the 

 Carnivora, in many Insectivora, and in some forms of the Artiodactyla. 



The third and final stayc in the metamorphosis of the premolar into 

 the molar pattern is reached by the addition of a fourth main element, 

 which Scott has called the tetartocone [Teraprof, fourth] ; it corresponds 



FIG. 199. Fourth upper and lower premolars of various Artiodactyla. After Scott. 



1. Dicotyles torquatus, family Dicotylida?. Molarization nearly complete. 



2. Tliinohyus lentus, family Dicotylida;. Molarization much less complete. 



3. Perclin i-iix /1,-iiin's, family Dicotylidse. 



4. TfiijfinoU ft> .- iii-iH-lii/xtomus, family Trigonolestidre, showing simple upper and lower premolar. 



in position and is analogous with the hypocone of the molars. As 

 observed by Scott, the tetartocone usually arises in the same manner 

 as the hypocone of the molars, namely, by the addition of a cusp at 

 the postero-internal angle of the crown immediately behind the 

 deuterocone (Fig. 199, No. 1). 



In the normal or usual evolution of the tetartocone behind the 

 deuterocone variations occur characteristic of different families of 

 mammals. Sometimes (in some Perissodactyla, <.//. Titanotheres) a 

 ridge extends back from the deuterocone, which splits in two, forming 

 the deuterocone anteriorly, the tetartocone posteriorly. 



This brief survey of the steps of premolar evolution proves that these 

 teeth follow an order of differentiation quite at variance with that 



