VI POSITION OF MARSUPIALS TO 
in JWyrmecobius (alone) is the third toe the longest; and no great 
difference can be detected between the’ third and fourth toes in 
the case of the genera Phascologale, Didelphys, and some others. 
Professor Leche compares the predominance of the fourth toe 
with the hyperphalangeal condition in the fourth toe of the embryo 
Crocodile, and considers it an archaic feature, not surpassed by 
the ancient characteristics of the Monotremata. Again it has been 
pointed out that in Phascologale and Perameles, the epistropheus 
(axis vertebra) has a separate rib as in Ornithorhynchus. In the 
third place, the likeness of the teeth of Myrmecobius to those of 
Ornithorhynchus is an argument in the same direction, which is 
furthermore supported by the great age (Mesozoic) of the Meta- 
therian group, if we are right in regarding those extinct creatures 
as Marsuptals. 
We may now mention certain facts which are not so generally 
used. The partly primitive structure of the right auriculo- 
ventricular valve in the Monotremata has no counterpart in 
any Marsupial which has been dissected; but there are traces 
in the latter of the characteristic “ ventral mesentery ” of Orni- 
thorhynchus and Echidna.' Mr. Caldwell’s interesting observation 
upon the segmenting egg of the Marsupial, the incompleteness 
of the first segmentation furrow (reminding us of the meroblastic 
ovum of the Monotreme), may possibly not turn out to be so 
exclusively Marsupial a feature as has been thought. 
The balance of evidence thus points to the nearer relationship 
of the Marsupials to the Eutherian mammals; and their great 
specialisation combined with certain evidences of degeneration 
(disappearance in part of the milk dentition), and their age, point 
to the fact that they are, at any rate, the descendants of an early 
form of Eutherian. But they must have separated from the 
Eutherian stock after it had acquired a definite diphyodonty and 
the allantoic placenta, the two principal features of the Eutherian 
as opposed to the Prototherian mammals. 
Nevertheless it seems probable that the Marsupial tribe is 
derived from some of the earliest Eutherians. And on this view 
may be explained the retention of Protothérian characters. 
The remaining Eutheria are obviously all to be referred to 
one great division with the possible exception of the Whales, 
whose affinities form one of the principal difficulties to the student 
' In Dendrolagus at any rate. See Proc. Zool. Soc. 1895, p. 132. 
