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TEETH OF THE VERTEBRATA. 423

terior external angle produced into a prominent process, which is con-
nected with the postero-external cusp by a sharp cutting ridge, The
cingulum also furnishes a broad ledge upon the outside of the two
external cusps.
In the three inferior true molars the median cone, the anterior wsal
lobe, and the internal tubercle are all well developed, and are disposed
in such a manner as to form an equilateral triangle, with the apex
directed forward and the base backward. Of these, the anterior basal
lobe occupies a position at the apex of the triangle, the median cone
and internal lobe being placed at the external and internal angles of
tlie base respectively. The posterior basal lobe is also present in the
form of a low heel, which may in some genera retain a simple cutting
form or may be broken up into several and become '' basin-shaped."
This form of tooth Prof. Cope proposed some years ago to designate by
the name of ^^ tuherculo-sectorial." There can be little doubt that it
furnishes the point of departure for the sectorial teeth of the lower jaw
Fig. 205.

Left Mandibular Ramus of Triisodnn quivirensis, three-fourths natural size, from the Puereo of Xew
Mexico ; «, external view, displaying last tempnrary molar in place ; h, the same from above; c, the
same, internal side, the temporary molar removed and the permanent Iburth premolar displayed
in ihe jaw ; d, the fourth premolar seen from above (after Cope).
of the modern Carnivora on the one hand, and the quadritubercular
lower molar of the entire unguiculate series on the other.
As will be seen, it displays the same elements that are found in the
inferior sectorial of the dog, the only difference being in the relatively