No. 410.] THE MAMMALIAN SUBCLASSES. 135 
There are still to be noticed certain phases of opinion 
which chiefly concern the relationships of the Monotremata. 
In a recent paper by Osborn (99) to which reference has 
already been made, we find the Monotremes separated off from 
the marsupials and placentals, and assigned an independent 
origin on a basis of their distinctness of organization. A 
similar view has also been expressed by Seeley ('96, '99). 
Under the circumstances this view has been a most natural 
one. The tendency of recent research has been essentially 
towards the revelation of placental characters in the Marsu- 
pialia (Wilson and Hill (97), Hill (97)), on the one hand, and 
the revelation of sauropsidan (theriodont, Seeley) characters 
in the Monotremata (Semon (94), Seeley (96,'99), van Bem- 
melen ('98,'99,'00), Smith (99), Sixta (00), Hochstetter (96), 
Ziehen (97). Thus the two groups have been naturally 
regarded as genetically distinct. It is probable, however, that 
in assigning a’ separate origin to them we overestimate their 
distance apart: in the first place, on account of the deceptive 
mask of placental characters which the Marsupialia, especially 
those of Australia, have assumed ; and in the second place, 
because, while diligent search has been made for sauropsidan 
characters in the Monotremata, little has been done towards a 
recognition of the prototherian characters of marsupials. That 
the latter field is not unproductive may be seen from the dis 
covery by Broom (97), in a foetal marsupial, of a distinct 
coracoid comparable to that of the Monotremata, and the 
recent results of McClure, showing that the numerous varia- 
tions of the postcaval and related veins in the opossum 
(Didelphys virginiana) are all modifications of a ground type 
which Hochstetter has shown to be characteristic of the 
embryonic Monotremata! Moreover, we should not lose sight 
of the familiar osteological characters which marsupials and 
monotremes have in common. 
Reviewing the evidence as a whole, there seems 
adequate reason for abandoning Huxley's hy pothesis. 
of the facts which have recently come to light are 
confirmatory than otherwise, and many others, which have been 
Professor McClure. 
ms to be no 
Many 
rather 
1 These results were kindly communicated to me by 
