458 
MR, O. THOMAS ON THE HOMOLOGIES AND 
Of the other Mammalian orders all fall easily enough into their places in this 
scheme,* with two exceptions, namely the Cetacea and Edentata. As regards the 
first, the general drift of the evidence seems to be that their ancestors have passed 
through a stage with a more or less complete milk dentition, which has gradually been 
again aborted,t its place being taken in the Odontocetes by the large and quasi- 
vegetative increase in the number of the molars, and in the Mystacocetes by the 
baleen, which latter has so fulfilled all the requirements of the animal that the “per¬ 
manent ” or original dentition has also been reduced to the position of a useless atavism, 
shed or absorbed before birth, and not playing any functional part in the life of the 
animal. 
In the Edentata, on the other hand, we find, as is well known, characteristics wholly 
at variance with those cf all other Mammals. In fact, a study of the teeth of this 
Stage. 
Premaxillary 
teeth. 
Fig. 5. 
Dentition. 
Maxillary teeth. 
Process. Remarks and examples. 
( ' 
1 
2 
3 
4 
-N 
5 
' 1 2 3 4 5 6 7 
8 
1 
9 
a 
V V V 
V 
V 
V V V V V V V 
V V 
ft 
o 
o 
o 
o 
V 
VVVVVVVV 
o 
Loss of i l 4 . 
7 
o 
o 
o 
o 
o 
V V V V V V V 
vl 
tv) 
Loss of i 5 . 
5 
o 
o 
o 
o 
o 
V V V V V V V 
V V . 
, &c.. up to 25 Increase of number 
of molars. 
C 
o 
o 
o 
o 
o 
V V V V V o o 
o 
o 
Decrease of number 
of molars. 
o 
o 
o 
o 
o 
o o o o o o o 
o 
o 
Total loss of teeth. 
‘1 
o 
o 
o 
o 
o 
V V V V V V V 
llfMf UWIf K 
V 
o 
Assumption of milk- 
teeth. 
Generalised Mam¬ 
mal (Stage I. of 
fig. 3). 
Dasypus. 
Xenurus, (fee. 
Priodon. 
Bradypodidae, 
Megatheriidas. 
Myrmecophagid®, 
Manidte. 
Tatusia. 
order soon induces a belief that the variance is so great as to preclude the possibility 
of the Edentates lying within the same lines of development as other Mammals, 
a belief that tallies exactly with the conclusions of Professor Parker,^ drawn from 
the embryology of the group. 
With this idea we may look upon the dentition of the Edentates as also based, like 
that of other Mammals, on Stage I. of the diagrams (fig. 3, and Plate 28), but modified 
in a different direction, and one peculiar to itself. Working out this suggestion, as 
before, by means of diagrams, we have the same Stage I. (fig. 5, a), in which the teeth 
are simple, and only divisible into five premaxillary and a variable number of maxillary 
* Except that, of course, innumerable problems still remain to be settled as to which of the typical 
number of teeth are present or have disappeared in the different groups. This is, however, merely a 
matter of detail, and does not affect the scheme in general. 
t Cf. Elowtor, ‘ Journ. Anat. Physiol.,’ vol. 3, 1869, p. 271. 
X ‘Phil. Trans.,’ 1885, p. 116. ‘Mammalian Descent,’ p. 97, 18S5. 
