1881,} 485 ave 
TRIISODON QUIVIRENSIS, gen. et sp. nov. Char. gen. Derived from the 
lower jaw. Probably only three premolars. True molars alike, consist- 
ing of three anterior cusps and a heel. The cusps are relatively small and 
the heel large. Of the former the internal is much smaller than the ex- 
ternal, and the anterior is rudimental, being merely a projection of the 
cingulum. The cutting edges of the large external cusp are obtuse. The 
heel is basin-shaped, and its posterior border is divided into tubercles, of 
which the external is a large cusp. The fourth premolar has no anterior 
inner tubercle, so that the anterior part of the crown consists of a com- 
pressed cutting cusp. The heel has two well-developed posterior cusps. 
The third premolar has a similar principal trenchant cusp, but a smaller 
heel. Canines large. 
This genus differs from Herpetotheriwm and Ictops in the simplicity of its 
fourth inferior premolar, and from Stypolophus and Deltatherium in the rudi- 
mental character of the accessory anterior cusps of the true molars, as well 
as in the three premolars. The rudimental anterior cusp of the true mo- 
lars, with the three similar true molars, separates it from Palwonyctis, and 
the presence of a conic inner cusp of the same indicates it as different 
from Amblyctonus and Periptychus. It is not possible to state whether 
Triisodon must be placed in the Amblyctonide or not, on account of the 
absence of the superior molar teeth. 
This specimen of the type species of this genus is instructive as showing 
the succession of premolar teeth. Both the third and fourth premolars 
have temporary predecessors. The predecessor of the fourth premolar 
differs much from it in form, and is essentially identical in all respects 
with the true permanent molars. The crown of the predecessor of ant 
third premolar is wanting, the roots only remaining in the jaw. 
The permanent third premolar was protruded before the permanent 
fourth. Which temporary tooth of Triisodon is homologous with the 
single one of the Marsupialia pointed out by Professor Flower?* As the 
additional permanent teeth of the placental Mammalia must have appeared 
later in time than the one already found in the implacentals, they must be 
those later protruded ; hence the fourth tooth in the jaw of Triisodon 
must be regarded as homologous with the fourth premolar of a placental, 
which is the last of that series to appear. If this be true, the tooth which 
follows the shed tooth of the Marsupials is not the fourth premolar, as 
supposed by Professor Flower, but the third premolar. This view is con- 
firmed by the fact that the milk tooth displaced by the fourth tooth in 
Triisodon resembles in all respects the true molars, just as the permanent 
tooth occupying the same position does in Didelphys and some extinct 
eocene genera. This goes to show that this tooth, permanent in marsu- 
pials, is temporary in placentals, and that, in spite of its form in the for- 
mer group, it is the fourth premolar, and not the first true molar, as sup- 
posed by Professor Flower. Thus the posterior milk-molar of diphyodonts 
is a permanent tooth in the Marsupialiu. 
*Trangactions of the Royal Society, 1867, p. 631. 
