454 
MR. O. THOMAS ON THE HOMOLOGIES AND 
represents a generalised Marsupial dentition with five incisors/'- four molars, and four 
premolars, of which the last is retarded, and has a milk-tooth superadded to it. The 
next advance on this in the Placental direction would be V., obtained by the retarda¬ 
tion of i 1 and the suppression of i 5 , a stage exactly represented in the abnormal Phas - 
IY. 
Y. 
VI. 
VII. 
VIII. 
IX. 
Fig. 2. 
Incisors. Canine. Premolars. 
7 2 3 4 5^ 11 2 3 7 
vvvvv y v v v 
V V V V o \ / V V V v 
V 
vv vvo \/vv vv 
V 
V V V 
o o 
V 
V 
V V V 
V 
V 
V 
o o 
V 
V 
V 
V 
V 
w 
HEP 
y 
w 
IIP 
w 
V 
V 
V o n 
v 
y 
V 
V 
V 
V 
“1®P' 
w 
w 
w 
ip 
IP 
mu' 
Molars. 
_A- 
1 2 3 4 5 
1_1 I _1 I_I I _ I O 
U U LJ U o 
U U U U o 
1_I 1_I I_I I_1 o 
!_I I_! I_I I_I o 
l_J LJ L_J o o 
cologale above described. Then, if the above theory is correct, should follow, first, 
VI.,+ where the retarded i 1 has developed a milk predecessor, and i 2 and pm 3 are 
retarded ; secondly, VII.,t in which the development of milk-teeth has extended from 
in front backwards and from behind forwards, and i 4 has dropped out ; and, thirdly, 
VIII., which would be just such a generalised Placental dentition as is now possessed 
teeth can be made out ■with certainty. But whatever changes the upper teeth have passed through 
must of necessity have also been undergone, pari passu , by the lower. 
* As in Didelphys and some Perameles. 
t Some of the ramifications of these intermediate stages may be represented by certain of the Eocene 
Creodonta of North America, as, for example, by Triisodon quivirensis, Cope, ‘ American Naturalist,’ 
vol. 15, 1881, p. 667 (figured op. cit., vol. 18, 1884, p. 257), which is said to have changed its two 
posterior premolars only, but the published descriptions of this interesting fossil and its allies are so 
incomplete and confusing that it is difficult to obtain any exact idea of their dental characters. It 
must also be noted that, although Professor Cope looks upon and argues from Triisodon as an animal only 
changing its pm 3 and pm 4 , there appears to be no evidence that it did not also change its pm 2 , canine 
and incisors, like Hycenodon and the Carnivora, of the present day. 
