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 Chrysochloris ; 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- 
‘Fic. 5.—Diagram of two upper and two lower left tubercular type of the 
quadritubercular molars in mutual apposition. The cusps . 
and ridges of the upper molars in double lines, and those bunodont serles. 
of the lower in black lines. The lower molars are looked In the upper molars 
at from below, asif transparent. pr, Protocone; hy,hypo- the primitive triangle in 
f=) 
cone ; pa, paracone ; me, metacone; ml, protoconule ; pl, ch 
metaconule ; prd, protoconid ; hyd, hypoconid ; pad, para- the secodont Series may 
conid ; med, metaconid ; end, entoconid. (After Osborn.) Temain 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, 
