TERTIARY MARSUPIALS 
the rocks as the Triassic, at the base of the Mesozoic series 
(dawn of the Age of Reptiles), are found in various parts of the 
world, including North America, a few jawbones Mesozoic 
and teeth indicating that some of the diminutive M@™™4s- 
forerunners of the Mammalia — able by reason of their mi- 
nuteness to run and hide and nibble in a world filled with 
ravenous reptiles — were marsupial in organization. Some of 
these early types lasted into or through the Jurassic period, but 
are not met with subsequently as fossils, except in Australasia 
and Patagonia. After a long interval marsupials again appear, 
in the Middle Tertiary rocks of the Old World, and soon after 
in similar formations in the New World. During the later 
Tertiary marsupials flourished, as did all other orders in that 
heyday of mammalian life. Such details of this part of their 
history as seem to be called for will be given when I come to 
speak of the families; at present I wish only to direct attention 
to the fact that in the past marsupials of all kinds were scattered 
all over the globe, whereas now, except the American opossums, 
no living member of the order is known outside of Australasia, 
where they constitute almost the entire mammalian fauna. 
This fact was very hard to explain until the discovery of many 
fossils supplied the key to a part, at least, of the puzzle. Dr. 
Lydekker ** has sketched this interesting matter concisely in 
the following sentences : — 
“Having their headquarters in Australia and New Guinea, where they 
form the dominant part of the mammalian fauna... the marsupials 
gradually diminish in number and importance in theislands of _ pjstripu- 
the Northwest, being largely mingled in Celebes and the neigh- _ tion. 
boring islands with types of mammals characteristic of the Indian or Oriental 
region. If, however, we pass westward across the deep channel separating 
the island of Celebes from Borneo, or its continuation which runs between 
Lombok on the east and Bali, at the extremity of Java, on the west, we shall 
find all the islands lying to the westward of that line devoid of marsupials 
and possessing a mammalian fauna akin to that of India. The line of the 
channel in question is, therefore, evidently a very important one as regards 
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