bunodont ([Greek: bounos], a hillock) type of molar into one in which
the four main tubercles, or columns, have assumed a crescentic form,
whence this type is termed selenodont ([Greek: selaenae], the new moon).
Further, there is the modification of the latter from a short-crowned,
or brachyodont type, to one in which the columns are tall, constituting
the hypsodont, or hypsiselenodont, type. It is noteworthy, however, that
in some instances there appears to have been a retrograde modification
from the selenodont towards the bunodont type, the hippopotamus being a
case in point. Other modifications are the loss of the upper incisors;
the development of the canines into projecting tusks; and the loss of
the anterior premolars.