Mr. Waterhouse, of the British Museum, after studying the annexed figs. 529. 531. and the descriptions of Prof. Plieninger, observes, that not only the double roots of the teeth and their crowns presenting several cusps, resemble those of Mammalia, but the cingulum also, or ridge surrounding the base of that part of the body of the tooth which was exposed or above the gum, is a character distinguishing them from fish and reptiles. "The arrangement of the six cusps or tubercles in two rows, in fig. 529., with a groove or depression between them and the oblong form of the tooth, lead him, he says, to regard it as a molar of the lower jaw. Both the teeth differ from those of the Stonesfield Mammalia[xiv-A], but do not supply sufficient data for determining to what order they belonged. Even in regard to the Stonesfield jaws, where we possess so much ampler materials, we cannot safely pronounce on the order." Professor Plieninger has sent me a cast of the smaller tooth, which exhibits well the characteristic mammalian test, the double fang; but Mr. Owen, to whom I have shown it, is not able to recognize its affinity with any mammalian type, recent or extinct, known to him. It has already been stated that the stratum in which the above-mentioned fossils occur is intermediate between the lias and the uppermost member of the trias. That it is really triassic may be deduced from the following considerations. In Wuertemberg there are two "bone-beds," one of great extent, and very rich in the remains of fish and reptiles, which intervenes between the muschelkalk and keuper, the other, containing the Microlestes, less extensive and fossiliferous, which rests on the keuper, or superior member of the trias, and is covered by the sandstone of the lias. The last-mentioned breccia therefore occupies the same place as the well-known English "bone-bed" of Axmouth and Aust-cliff near Bristol, which is shown[xv-A] to include characteristic species of muschelkalk fish, of the genus Saurichthys, Hybodus, and Gyrolepis. In both the Wuertemberg bone-beds these three genera are also found, and one of the _species_, Saurichthys Mougeotii, is common to both the lower and upper breccias, as is also a remarkable reptile called Nothosaurus mirabilis. The Saurian called Belodon by H. Von Meyer of the Thecodont family, is another Triassic form, associated at Diegerloch with Microlestes. Previous to this discovery of Professor Plieninger, the most ancient of known fossil Mammalia were those of the Stonesfield slate, a subdivision of the Lower Oolite[xv-B] no representative of this class having as yet been met with in the Fuller's earth, or inferior Oolite (see Table, p. 258.), nor in any member of the lias.