416 PROCEEDINGS OF THE ACADEMY OF [1892. 



however, it cannot be homologous with that element, its place with 

 reference to the metaconid being entirely different. Its homologies 

 are rather with the tetartocone of the upper premolar, as will appear 

 when it is remembered that there is not that reversal in the position 

 of the cusps of the inferior premolars compared with the superior 

 ones which obtains between the upper and lower molars, the primary 

 cusp or protocone remaining upon the external side of the crown 

 in both upper and lower premolars. The tetartoconid is usually 

 the last cusp to make its appearance upon the crown of the inferior 

 jDremolar, but there are exceptions to this rule. For example, in 

 Trilsodon p* consists of a very large protoconid and a lower but yet 

 large heel or talon, made up of the metaconid upon the outside 

 and the tetartoconid on the inside ; neither para- nor deuteroconid 

 is present. 



In a former paper (No. 11, pp. 48-9) I gave a somewdiat different 

 account of the evolution of the lower premolar, in that I regarded 

 the talon of the premolars as equivalent to that of the molars, and 

 like it composed of hypo- and entoconids. This seemed to follow 

 from the fact that the position of the cusps which make up the talon 

 with reference to the protoconid is the same in both classes of teeth. 

 An examination of a more extended series of premolars has, how- 

 ever, convinced me that the external element of the premolar talon 

 is the equivalent of the metaconid and not the hypoconid of the 

 molar. If this be true, the internal element obviously cannot be 

 the entoconid, while its relation to the tetartocone of the superior 

 premolar is the same as that of the deuteroconid to the deutero- 

 cone. 



If the foregoing description be accepted as correct, it will appear 

 that, while the inferior premolars may contain three elements 

 which are homologous with molar cusps, the upper premolars con- 

 tain but one cusp which can be homologized with a molar cusp, 

 and, furthermore, that while in the molar crown the protocone is 

 on the inner, and the protoconid on the outer side, in both sets of 

 premolars, upper as well as lowei", it retains its primitive position 

 upon the external side. 



The following table will serve to exhibit the correspondences of 

 position (not of homology) between the molar and premolar 

 elements when all are present. 



