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Not single specimen of any higher, or placental, mammal has yet been found in the whole Mesozoic Era. We must, however, beware of simply transferring to the Mesozoic world the kinds of Monotremes and Marsupials which we know in nature to-day.
From the comparative anatomy and ontogeny of the existing Marsupials we may draw very interesting conclusions as to their intermediate position between the earlier Monotremes and the later Placentals. Moreover, all the Marsupials have teats on the mammary glands, at which the new-born animal sucks.
The corpus callosum, which forms a sort of wide bridge connecting the two hemispheres of the cerebrum, is only fully formed in the Placentals; it is very rudimentary in the Marsupials and Monotremes.
They are placed by the mother in a marsupial pouch on her ventral surface, and here nourished till able to care for themselves. Pardon a moment's digression. The marsupials, except the opossum, are confined to Australia, and the oviparous mammals, or monotremes, to New Zealand.
In 1894 Richard Semon further proved that these large eggs, rich in food-yelk, have a partial segmentation and discoid gastrulation, as I had hypothetically assumed in 1879; here again they resemble their reptilian ancestors. The construction of the mammary gland is also peculiar in the Monotremes.
It is more difficult to identify Monotreme remains, but the fact that Monotremes have survived to this day in Australia, and the resemblance of some of the Mesozoic teeth to those found for a time in the young Duckbill justify us in assuming that a part of the Mesozoic mammals correspond to the modern Monotremes.
Some zoologists have lately advanced the opposite opinion, that the Marsupials represent a completely independent sub-class of the Mammals, with no direct relation to the Placentals, and developing independently of them from the Monotremes.
Over the earth generally, however, they were superseded by the placental mammals, which suddenly break into the geological record in the early Tertiary, and spread with great vigour and rapidity over the four continents. Were they a progressive offshoot from the Mesozoic Marsupials, or Monotremes, or do they represent a separate stock from the primitive half-reptile and half-mammal family?
The fore-brain or cerebrum, in particular, is so small that it does not cover the cerebellum. Like these, the Monotremes have a strongly developed caracoideum. From these and other less prominent characteristics it follows absolutely that the Monotremes occupy the lowest place among the Mammals, and represent a transitional group between the Tocosauria and the rest of the Mammals.
The lowest forms of the Mammals resemble Birds in many ways. Some of them are toothless, and many of them have the same primitive intestinal arrangements possessed by the birds, from which arises their name, Monotremes. These Monotremes may be called half-bird and half-mammal.
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