These primitive mammals help us to reconstruct the mammal life of the Mesozoic Epoch. The bones that we have are variously described in geological manuals as the remains of Monotremes, Marsupials, and Insectivores. Many of them, if not most, were no doubt insect-eating animals, but there is no ground for supposing that what are technically known as Insectivores (moles and shrews) existed in the Mesozoic. On the other hand, the lower jaw of the Marsupial is characterised by a peculiar hooklike process, and this is commonly found in Mesozoic jaws. This circumstance, and the witness of Australia, permit us, perhaps, to regard the Jurassic mammals as predominantly marsupial. 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. 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. In some of the excellent "restorations" of Mesozoic life which are found in recent illustrated literature the early mammal is represented with an external appearance like that of the Duckbill. This is an error, as the Duckbill has been greatly modified in its extremities and mouth-parts by its aquatic and burrowing habits. As we have no complete skeletons of these early mammals we must abstain from picturing their external appearance. It is enough that the living Monotreme and Marsupial so finely illustrate the transition from a reptilian to a mammalian form. There may have been types more primitive than the Duckbill, and others between the Duckbill and the Marsupial. It seems clear, at least, that two main branches, the Monotremes and Marsupials, arose from the primitive mammalian root. Whether either of these became in turn the parent of the higher mammals we will inquire later. We must first consider the fresh series of terrestrial disturbances which, like some gigantic sieve, weeded out the grosser types of organisms, and cleared the earth for a rapid and remarkable expansion of these primitive birds and mammals.