668 General Notes. [ August, 
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 isa large cusp. The fourth premolar has no ante- 
rior inner tubercle, so that the anterior part of the crown consists 
of a compressed cutting cusp. e heel has two well-developed 
posterior cusps. e third premolar has a similar principal tren- 
chant cusp, but a smaller heel. Canines large. 
This genus differs from Herpetotherium and Ictops in the sim- 
plicity of its fourth inferior premolar, and from Szypolophus and 
Deltatherium in the rudimental character of the accessory anterior 
cusps of the true molars, as well as in the three premolars. - The 
rudimental anterior cusp of the true molars, with the three similar 
true molars, separates it from Paleonyctis, and the presence of a 
conic inner cusp of the same indicates it as different from Amblye- 
tonus and Periptychus. It is not possible to state whether 77zso- 
don must be placed in the Amdélyctonide 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 essen- 
tially identical in all respects with the true permanent molars. 
e crown of the predecessor of the third premolar -is wanting, 
the roots only remaining in the jaw. 
The permanent third premolar was protruded before the per 
manent fourth. Which temporary tooth of Zriisodon is homolo- 
gous with the single one of the Marsupialia pointed out by Pro: 
fessor Flower?! As the additional permanent teeth of the pla- 
cental Mammalia must have appeared later in time than the one 
already found in the implacentals, they must be those later pro- 
truded; hence the fourth tooth in the jaw of 7riisodon must be = 
garded 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 a 
fourth premolar, as supposed by Professor Flower, but the oe 
premolar. This view is confirmed by the fact that the milk too 
displaced by the fourth tooth in Zriisodon resembles 1n all he 
spects the true molars, just as the permanent tooth occupying the 
same position does in Didelphys and some extinct eocene gener’ 
This goes to show that this tooth, permanent in marsupials “4 
temporary in placentals, and that, in spite of its form in the ith 
mer group, it is the fourth premolar, and not the first true Mo ar, 
as supposed by Professor Flower. Thus the posterior milk-mo ar 
of diphyodonts is a permanent tooth in the Marsupialia. se 
This observation confirms my conclusion that the Creodon : 
form a group intermediate between the JMJarsupialia and Car 
1 Transactions of the Royal Society, 1867, p. 631. 
