32 



GENERAL ANATOMICAL CHARACTERS 



4, 6) having one inner and two outer cusps, while the reverse 

 condition obtains in those of the lower jaw (Fig. 4, 7). These 

 three cusps of the simple tritubercular tooth are collectively desig- 

 nated as the primitive triangle ; in the upper tooth the inner cusp 

 is termed the protocone, the antero-external one the paracone, and 

 the postero-external the metacone ; the corresponding cusps of the 

 lower tooth being named protoconid, paraconid, and metaconid — 

 the protoconid being here on the outer side of the crown. 



It is thus apparent that in the first, or haplodont type, as well 

 as in the triconodont type, the upper and lower molars are alike ; 

 while in the simple tritubercular type they have a similar pattern, 

 but with the arrangement of the cusps reversed. This simple 

 tritubercular type occurs in the Mesozoic genus Spalacotherium 

 (Fig. 4, 6 and 7), and apparently in the existing Chrysochhris ; but 

 in the majority of tritubercular forms, while this primitive triangle 

 forms the main portion of the crown, other secondary cusps are 

 added, the homologies of which in the upper and lower teeth are 

 somewhat doubtful. At the same time that we have the addition 

 of these secondary cusps we also find trituberculism differentiating 

 into a secodont and a bunodont series, according as to whether the 

 dentition becomes of a cutting or a crushing type. 



Thus in the lower molars (Fig. 4, s and 9) we very frequently 

 find the three cusps of the primitive triangle elevated and connected 

 by cross crests, while there is an additional low posterior heel or 

 talon, which may be termed the hypoconid. This tubercular- 

 sectorial sub-type, as it is termed, is found in the lower molars of 

 many Polyprotodont Marsupials and Insectivores, and it also occurs 

 in the lower carnassial teeth of the true Carnivora. The presence 



of two cusps (inner and 



outer) to the talon con- 

 verts this modification 

 into a quinquetubercular 

 form ; while, by the sup- 

 pression of one of the 

 three primitive cusps, it 

 develops into the quadri- 

 tubercular type of the 

 bunodont series. 



In the upper molars 

 the primitive triangle in 

 the secodont series may 

 remain purely tricuspid ; 

 but the addition of in- 

 termediate cusps, both in the secodont and bunodont series, may give 

 rise to a quinquetubercular type ; these intermediate cusps being 

 respectively designated as the protoconule and metaconule (Fig. 5, 



Fig. 5. — Diagram of two upper and two lower left 

 quadritubercular molars in mutual apposition. The cusps 

 and ridges of the upper molars in double lines, and those 

 of the lower in black lines. The lower molars are looked 

 at from below, as if transparent, pr, Protocone ; hy, hypo- 

 cone ; pa, paracone ; me, metacone ; ml, protoconule ; pi, 

 metaconule ; prd, protoconid ; hyd, hypoconid ; pad, para- 

 conid ; med, metaconid ; end, entoconid. (After Osborn.) 



