Next anterior in age are the mammalia of the Lower Oolite of Stonesfield, of which four species are known, also very small, and probably marsupial, with one exception, the Stereognathus ooliticus, which, according to Professor Owen's conjecture, may have been a hoofed quadruped and placental, though, as we have only half of the lower jaw with teeth, and the molars are unlike any living type, such an opinion is, of course, hazarded with due caution.
Still older than the above are some fossil quadrupeds of small size, found in the Upper Trias of Stuttgart in Germany, and more lately by Mr. C. Moore in beds of corresponding age near Bristol, which are also of a very low grade, like the living myrmecobius of Australia. Beyond this limit our knowledge of the highest class of vertebrata does not as yet extend into the past, but the frequent shifting back of the old land-marks, nearly all of them once supposed in their turn to indicate the date of the first appearance of warm-blooded quadrupeds on this planet, should serve as a warning to us not to consider the goal at present reached by palæontology as one beyond which they who come after us are never destined to pass.
On the other hand, it may be truly said, in favour of progression, that, after all these discoveries, the doctrine is not gainsaid, for the less advanced marsupials precede the more perfect placental mammalia in the order of their appearance on the earth.
If the three localities where the most ancient mammalia have been found,—Purbeck, Stonesfield, and Stuttgart—had belonged all of them to formations of the same age, we might well have imagined so limited an area to have been peopled exclusively with pouched quadrupeds, just as Australia now is; while other parts of the globe were inhabited by placentals, for Australia now supports one hundred and sixty species of marsupials, while the rest of the continents
