INTRODUCTION 



19 



molars became necessary ; and this requirement was effected by the introduction 

 in the lower jaw of a usually triconid talonid to function as antagonist against 

 the principal cone of the corresponding upper molar. At the same time, in 

 the upper dentition, the two outer cones became larger, and, in addition, two 



Fig. 23. 



Miiltitubercular molars of Allotheria. A, B, Upper 

 tooth of Cimolomys. C, Lower tooth of Cimolomys. 

 3/1. (After Marsh.) 



Fig. 24. 

 Polylophodont molar of Stegodon. 



About 1/2- 



minor cusps were formed. Of these latter, the hindermost often became 

 metamorphosed, when the diet was mixed, into an intermediate cusp, although 

 this may also arise independently as a growth from the cingulum. 



The following nomenclature is applied to the different elements of molars 

 having four or five cusps : 



A. Upper Tritubercular Molars. 



Anterior inner cone or cusp 



protocone 



pr. 



Posterior „ „ „ 



hypocone 



i^y- 



Anterior outer „ „ ... 



. paracone 



pa. 



Posterior „ „ „ 



. metacone 



me. 



Anterior intermediate cone or cusp . 



protoconule 



pi. 



Posterior „ „ „ . . 



metaconule 



ml. 



Antero-external peripheral cusp 



parastyle 



ps. 



Medio-external cusp .... 



mesostyle 



ms. 



Postero-external cusp .... 



metastyle 



mts. 



B. Lower Tritubercular-Sectori 



al Molars. 





Anterior outer cone or cusp 



protoconid 



prd. 



Posterior „ „ ,, 



hypoconid 



hyd. 



Anterior unpaired cone .... 



paraconid 



pad. 



Posterior ,, .... 



hypoconulid 



hyld. 



Anterior inner cone or cusp . . . . 



metaconid 



med. 



Posterior ,, „ ,, 



eutoconid 



end. 



The classification of mammals has been very greatly influenced by the 

 discovery of numerous transitional fossil forms, since genera and even families 

 and orders of recent forms, which apparently are not allied, become interrelated 

 by means of fossil ancestors, and the presumably well-defined limits between 

 them become less distinct. On the other hand, fossil forms have been found 

 which require the establishment of new families, and even, in some cases, a new 

 order, to accommodate genera absolutely unique among mammals. In most 

 cases, the evolutionary history of single genera, families and orders can be 

 traced with more certainty among mammals than in any other class of 



