Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 Ch. XX.] LOWER PURBECK. 389 inhabited by placentals, for Australia now supports one hundred and sixty species of marsupials, while the rest of the continents and islands are tenanted by about seventeen hundred species of mamma- lia, of which only forty-six are marsupial, namely, the opossums of North and South America. But the great difference of age of the strata in each of these three localities seems to indicate
Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 Ch. XX.] LOWER PURBECK. 389 inhabited by placentals, for Australia now supports one hundred and sixty species of marsupials, while the rest of the continents and islands are tenanted by about seventeen hundred species of mamma- lia, of which only forty-six are marsupial, namely, the opossums of North and South America. But the great difference of age of the strata in each of these three localities seems to indicate the pre- dominance throughout a vast lapse of time (from the era of the Upper Trias to that of the Purbeck beds) of a low grade of quadru- peds ; and this persistency of similar generic and ordinal types in Europe while the species were changing, and while the fish, reptiles, and mollusca were undergoing vast modifications, would naturally lead us to suspect that there must also have been a vast extension in space of the same marsupial forms during that portion of the sec- ondary epoch which has been termed ' the age of reptiles.' Such an inference as to the wide geographical range of the marsupials of the olden time has been confirmed by the discovery in the Trias of North America of three lower jaws of a quadruped allied to Myrmecobius. It was found by the late Dr. Emmons in beds probably coeval with the ' Keuper ' of Europe. The predominance in earlier ages of these mammalia of a low grade, and the absence at present of species of higher organization, is certainly in favor of the theory of progressive development. Beneath the freshwater strata to which the mammaliferous marl belono-s is a thin band of greenish shales, with marine shells and im- pressions of leaves, like those of a large Zostera, forming the base of the Middle Purbeck. Lower Purbech.—Beneath the thin marine band last mentioned, purely freshwater marls occur, containing species of Cypris (fig. 375, a, 6), Va
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