1897.] Trituberculy : 1007 
relations of the upper and. lower crowns in apposition, as in 
the evolution of the sectorial and lophodont types. As to the 
form of the cusps, we pass from simple pointed cusps to three 
well known modes of modification to which the adjective 
“bunoid,” “lophoid,” and “ selenoid” may be applied. A 
combination of these terms gives us a permanent system of 
distinguishing the complex forms of ungulate molars from 
each other, by referring first to the form of the protocone; 
second, to that of the outer paracone and metacone. Thus in 
Palzxosyops, as the protocone is bunoid and the outer cusps are 
selenoid, the crown may be distinguished as “ buno-seleno- 
dont.” In Palæotherium the protocone is “ lophoid,” and it may 
be described as “lopho-selenodont.” Rhinoceros is truly 
“lophodont,” since all its six cusps are “ lophoid.” These are 
preferable to the terms “ tapirodont,” “symborodont,” “ bath- 
modont,” “loxolophodont,” etc., proposed by Cope, because 
the latter are associated with generic types. 
THE EVOLUTION OF THE UNGULATE MOLAR. 
The fact of derivation of all ungulate molars (excepting in 
the Amblypoda) from sexitubercular upper and lower crowns, 
leads us to look sharply for traces of these six tubercles from 
the primitive plan of Euprotogonia. These six cusps are almost 
invariably found in the upper molars of both perissodactyls 
and artiodactyls up to the middle of the Eocene period, as ty- 
pified in Hyracotherium and Homacodon or Dichobune. In the 
lower molar the trigon loses the “ paraconid ” and the talon 
loses the “ hypoconulid,” the latter persisting only in the last 
molar as the “third lobe.” This loss was accompanied by the 
complete transformation of the lower molars from the “ seco- 
dont ” to the comparative “ bunodont” type, as effected in the 
lowering of the “ trigonid ” to the level of the “talonid.” This 
is exemplified in the steps between the first and third molars 
of the creodont genus Miacis (Fig. 8). In a side view of 
all early ungulate molars, such as Hyracotherium, we see that 
the “ trigonid” is still the highest portion of the crown. In 
the ungulates, unlike the carnivores, all three molars were 
affected simultaneously. An exactly similar levelling pro- 
