452 MAMMALIAN ANATOMY 



that element, but rather with the deuterocone of the upper premolar, 

 and may consequently be called the deuteroconid (Fig. 370, V). The 

 latter element also varies as to the relative time of its appearance ; 

 sometimes it is the only element present in addition to the protoconid 

 (e.g., Pelycodus, Chriacus, Protogonia, Fig. 370, S), or it may be de- 

 veloped after either the paraconid or the metaconid, or it may appear 

 last of all, and in very many cases it is altogether absent, Yet when 

 it does appear, its homologies are perfectly obvious. . A fifth element 

 is sometimes added to the premolar crown posterior to the protoconid 

 and interior to the metaconid, thus occupying the position held by 

 the entoconid of the molars. Clearly, however, it cannot be homol- 

 ogous with that element ; its place with reference to the metaconid is 

 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 pre- 

 molars 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 pre- 

 molars. 



" The following table will serve to exhibit the correspondences of 

 position (not of homology) between the molar and premolar elements 

 when all are present : 



UPPER JAW. LOWER JAW. 



MOLAR. PREMOLAR. MOLAR. PREMOLAR. 



Protocone = Deuterocone. Protoconid = Protoconid. 



Paracone = Protocone. Paraconid = Paraconid. 



Metacone = Tritocone. Metaconid = Deuteroconid. 



Hypocone = Tetartocone. Hypoconid = Metaconid. 



Entoconid = Tetartoconid. 1 " 



Before studying the value of the variations in teeth as evidence 

 of the evolution of tooth-forms, we must put aside abnormalities of 

 structure due to disease or to early disturbances of the tooth germs, 



1 These names for the dental cusps are compounds of conus, a cone, and prefixes 

 designating their relative position or relative order of appearance : protos, the 

 first; deuteros, the second; tritos, the third; tetartos, the fourth ; para, beside, along- 

 side; meta, behind, after; hypo, under; entos, within, to the inner side. The suffix 



' 



-id denotes a cusp on a lower tooth. 



