194 EVOLUTION OF MAMMALIAN MOLAR TEETH 



The cingulum generally embraces the entire inner face of the crown, 

 forming anterior and posterior cingulum cusps or cingules, which are 

 characteristic of the insectivorous forms, while in the supposed car- 

 nivorous and omnivorous forms, distinct basal cusps rise posteriorly and 

 sometimes anteriorly [Triconodon, Figs. 8, 195 4] above the cingulum. As 

 in the latter genera the cingulum is present with the basal cusps, it 

 probably precedes them in evolution, but there is no direct evidence 

 (in the Triconodonta) of the conversion of ciiinulcx into true basal cusps, 

 such as we find in the molars. 



A review of the premolars of all the genera shows that they are 

 sharply distinguished from the incisors and from the molars, and less 

 distinctly from the canines in many instances. 1 In several genera they 

 have undergone considerable specialization, as in the production into lofty 

 cones of pm^ of Achyrodon [Fig. 195 1J], or the apparently incipient 

 assumption of the molar pattern in Kurt od on [Figs. 195 15]" 



2. ADAPTATION OF PREMOLARS. 



The premolar teeth in general are quite as adaptive and independent 

 in evolution as the molars. While in many families of mammals the 

 first, second, and third, and more rarely the fourth promolars retain more 

 or less of this simple ancestral structure ; in other families the premolars 

 become greatly complicated. They either (a) enter upon an especial 

 adaptive evolution of their own, as for example in the upper sectorials 

 of the Cats (Felicia- 1 ), or the elaborate fourth premolars of the Plagiau- 

 lacida? (pp. 102, 106), or (b) by a serial analogous development they more 

 or less closely mimic the structure and supplement the exact functions 

 and uses of the molar teeth : this mimicry reaches its highest extreme 

 among the Perissodactyl or odd-toed Ungulates, such as the horses, where 

 the premolars gradually metamorphose into the molar pattern and even 

 become superior to the molars in size and complication. 



3. VARIOUS UPPER PREMOLAR TYPES. 



1. Persistence of a simple conical form throughout. Realized in the 

 inferior premolars of Dromatlurium (Fig. 3). 



2. The posterior or 4th superior premolar becomes sectorial, the 

 anterior premolars remaining more or less simple and conical, e.g. most 

 Carnivora. 



3. The 4th, 3rd, and 2nd superior premolars become more or less 

 uniformly bicuspid, <\y. most Primates. 



1 There is strong support among the Jurassic mammals and the recent Insectivora for 

 the opinion first expressed by E. Ray Lankester, that the canine is originally a bifanged 

 tooth and represents a modified anterior premolar. 



