1896. ] MAMMALIAN DENTITION. 581 
with those teeth, but differed from them in being retarded in its 
development. 
While investigating the development of dpm. 4 and ppm. 4 in 
the Insectivora, I have kept the above conclusion in mind, and 
allowing for the differences in the condition of the dentition in these 
two groups (Insectivora and Diprotodont Marsupials) I find a 
strong confirmation of this view, that ppm. 4 represents a tooth 
originally situated in front of dpm. 4, but retarded in its 
development, and subsequently displaced backwards or overgrown 
by dpm. 4. 
This condition is more marked in the upper jaw, where in three 
of the genera investigated ppm. 4 develops distinctly in front of 
dpm. 4, in two slightly so, while only in one does it develop distinctly 
lingual to dpm. 4 (this is in Sorew probably a. specialized form). 
The molariform condition of dpm. 4 is well marked, but while in 
some Insectivora ppm. 4 is distinct in pattern, in others it is also 
molariform—the former condition being more marked in other 
groups of mammals, in some of which (Carnivora and Marsupials) 
ppm. 4 is so distinct in the characters of its crown from its 
predecessor that, taken in connection with the developmental 
features above recorded, I am forced to the conclusion that dpm. 4 
is a true molar accelerated in its development and growing forwards 
over the top of the retarded true 4th premolar, or, in other words, 
dpm. 4 is the only true deciduous molar, while the tooth usually 
termed ppm. 4 is really the milk, but non-deciduous 4th premolar. 
The above would account for the striking differences in character 
between the supposed deciduous and permanent 4th premolars of 
the “ Kangaroo Rats,” where dpm. 4 is molariform, and ppm. 4 
that marvellous compressed cutting-tooth, identical in pattern with 
the anterior premolar dpm. 3. So also in the Carnivora with regard 
to the upper carnassial tooth. I think it is easier to conceive that 
the anterior molar should be accelerated in its development in order 
to supply the young animal with a crushing-tooth, than to believe 
with Cope (2) that the mere fact of a tooth-germ being shifted in 
its position relative to the angle of the mouth would cause such a 
total change in the character of two tooth-germs which were 
supposed to develop side by side as sisters from the same region of 
the dental lamina. 
It is only fair to state that Leche (9. pp. 103 and 139) after 
considering the views put forward by me in a former paper (28), 
still concludes that the successor to dpm. 4 is the true represen- 
tative of that tooth in the permanent series. 
The Molars. 
I have already described in my detailed account of the 
development of the molar teeth the presence of outgrowihs 
from the dental lamina, to which structure the enamel-organs on 
these teeth are attached and from which they have arisen, both of 
the labial and lingual side of these teeth; these outgrowths, though 
