1892.] NATURAL SCIENCES OF PHILADELPHIA. 405 



THE EVOLUTION OF THE PREMOLAR TEETH IN THE MAMMALS. 



BY W. B. SCOTT. 



The mode of evolution of the mammalian molar tooth up to and 

 from the tritubercular pattern, which has been elaborated by Cope 

 (No. 1) and Osborn (No. 7) as well as the nomenclature proposed 

 by the latter for the various cusps or elements which make up the 

 crown of the mammalian molar, has been very generally accepted 

 by morphologists. According to this view, the primitive form of 

 tooth in both jaws is that exemplified by many reptiles, viz., a per- 

 fectly simple cone. The cusp which represents this primordial 

 element throughout the various phyla of mammals is called the 

 protoeone. To this are added, as a second stage, small anterior and 

 posterior cusps, called respectively the para- and metaeones. This 

 stage gives a crown composed of three cusps all in the same antero- 

 posterior line and is exemplified by such genera as Dromaiherium 

 and Trkonodon. The next step would appear to consist in a shift- 

 ing of the relative position of the cusps, and now the molars of the 

 two jaws, hitherto similar, begin to display an important difference. 

 In the upper jaw the protoeone begins to move inward or mesially, 

 the para- and metaeones remaining upon the external or lateral 

 side of the crown, while in the lower jaw the protoconid remains 

 upon the outer side of the tooth, the para- and metaconids shifting 

 toward the inner side. In this stage the molar crowns have a tri- 

 angular shape, the protoeone forming the apex, and the para- and 

 metaeones the base ; but while in the superior molar the apex is 

 directed mesially, in the inferior tooth it is directed laterally. 

 The next stage affects primarily the lower molars and depends 

 upon the change from the primitive conditio^, in which the upper 

 and lower molars merely pass each other with a shearing move- 

 ment, or in which each tooth opposes merely one tooth in the oppo- 

 site jaw, to the alternating condition, in Avhich each lower molar 

 opposes two upper ones, or opposes one and shears against another. 

 The change consists in the addition to the primitive triangle of the 

 tooth (trigonid) of a low heel or talon, which may subsequently 

 divide into two cusps, of which the outer is called the hypoconid 

 and the inner the entoconid. It may, however, be the case that the 

 hypoconid is first devel()i)ed and the entoconid subsequently added 



