1897,] Trituberculy : 1009 
1°. The addition of one or more peripheral cusps or “ styles ” 
as upgrowths from the cingulum. ‘These reached their most 
extreme development in the Equidæ. (See Fig. 10.) 
2°. The persistence or degeneration of the cingulum at cer- 
tain points, for all primitive molars are completely invested by 
a broad cingulum. 
3°. The moletling of the cusps into the “bunoid,” “ lophoid” 
or “selenoid ” form. 
4°. The metatrophic or unequal growth of the cusps, espe- 
cially as affecting the external pair, ee and metacone, 
in the upper molars. 
5°. The shifting of the cusps kom their primitive position 
upon the crowns. 
6°. The shifting point of union of these transverse crests 
with the external crest. 
The differential features of the development of ungulate 
molars all group around these six heads. If we were examin- 
ing an isolated molar tooth from the lower Eocene, the first 
step would be to locate its primary cusps and then note its 
divergence as tested by the above differentia. We would then 
be in a position to make a conjecture as to the series in which 
this molar belonged—as no two series are modified similarly 
in all these respects. Yetthe prevailing method among many 
paleontologists is to pass lightly over most of the differentia 
and, for example, group widely divergent forms under the 
Lophiodontide as if in the constitution of these dense enamelled 
tissues nature could lightly pass from one to another. 
A few words now upon the secondary “styles.” Their func- 
tion is evidently to increase and elaborate the crushing surface 
of the crown. In Phenacodus the first to appear is the “ meso- 
style ” between the paracone and metacone, but this genus was 
on a side line of the Condylarthra. In all true perissodactyls 
and artiodactyls, the first peripheral cusp to appear is the an- 
tero-external buttress of the upper molars, which we call the 
“ parastyle,” since it adjoins the paracone. The “mesostyle ” 
appears later, and only in those ungulates in which the para- 
cone and metacone are moulded into crescents. Thus the ~ 
lower Eocene Hyracotherium does not exhibit this cusp, but it 
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