EVOLUTION OF THE PREMOLARS 195 



4. The 4th superior premolar more or less completely transforms 

 into the molar pattern, the third, second, and first remaining simpler 

 (e.g. some Artiodactyla, as Agriochcerus). 



5. The 4th and 3rd superior premolars transform into the molar 

 pattern, the first and second adopting an entirely different order of 

 evolution (e.g. Galeopithecus). 



6. The 4th, 3rd, and 2nd premolars successively partially transform 

 in the molar pattern. Example, many Perissodactyla, such as the 

 Titanotheres and lihinocerotidas. 



7. The 4th, 3rd, and 2nd premolars completely transform into the 

 molar pattern. Example, some Perissodactyla (Equiclee). 



I This premolar metamorphosis into the molar pattern observed in types 

 4, 5, 6, 7, above is a very gradual process, requiring hundreds of 

 thousands if not millions of years, and from the biological standpoint 

 most interesting as illustration of convergence, because form exactly 

 similar to that of the molars is finally attained from somewhat dis- 

 similar beginnings. 



4. CUSP ADDITION IN THE PREMOLARS. 



The first broad and systematic treatment of the subject of premolar 

 evolution was that by Professor W. B. Scott. 1 It is somewhat too special 

 to be cited in full here. We accept Scott's interpretation in full as 

 regards the upper premolars, but have adopted a different interpretation 

 of the evolution of the lower premolars. 



SUPERIOR PREMOLARS. 



It is important to note here that all of the following description is 

 based on the older Cope-Osborn theory that the superior premolars 

 have followed a different order of cusp addition from the molars ; this 

 is now met by the newer theory (see Chapter IX.) that the premolars 

 follow practically the same order of cusp addition as that originally 

 followed by the molars. Pending the solution of this question the 

 comparisons which follow are made on the basis of the older theory. 



We may take the progressive complication of the fourth superior 

 premolar as a standard ; the order of succession of the cusps in this 

 tooth is rather constant, while in the more anterior premolars there are 

 more various modes of complication. 



First stage. As early as the Basal Eocene period the fourth upper 

 premolar, in every known genus in which the premolars tend to imitate 



1 " The Evolution of the Premolar Teeth in Mammals," Proc. Acad. Nt. Sci. Phila., 

 1892, pp. 40.1-444. 



