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 Würtemberg 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 Würtemberg 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.
Thecodont Saurians.—This family of reptiles is common to the Trias and Permian groups in Germany, and the geologists employed in the government survey of Great Britain have come to the conclusion, that the rock containing the two species alluded to at [p. 306.], and of which the teeth are represented in [figs. 348], [349.], ought rather to be referred to the Trias than to the Permian group.
CRETACEOUS GASTEROPODA.
In speaking of the chalk of Faxoe in Denmark ([p. 210.]) or the highest member of the Cretaceous series, I have remarked that it is characterized by univalve Mollusca, both spiral and patelliform, which are wanting or rare in the white chalk of Europe. This last statement requires, I find, some modification. It holds true in regard to certain forms, such as Cypræa and Oliva, found at Faxoe; but M. A. d'Orbigny enumerates 24 species of Gasteropoda from the white chalk (Terrain Sénonien) of France alone. The same author describes 134 French species of Gasteropoda from the chloritic chalk marl and upper greensand (Turonien), 77 from the gault, and 90 from the lower greensand (Neocomien), in all 325 species of Gasteropoda, from the cretaceous group below the Maestricht beds. Among these he refers 1 to the genus Mitra, 17 to Fusus, 17 to Trochus, 4 to Emarginula, and 36 to Cerithium. Notwithstanding, therefore, the peculiarity of the chambered univalves of various genera, so abundant in the chalk, the Mollusca of the period approximate in character to the tertiary and recent Fauna far more than was formerly supposed.
DICOTYLEDONOUS LEAVES IN LOWER CRETACEOUS STRATA.
M. Adolphe Brongniart when founding his classification of the fossiliferous strata in reference to their imbedded fossil plants, has placed the cretaceous group in the same division with the tertiary, that is to say, in his "Age of Angiosperms."[xvi-A] This arrangement is based on the fact, that the cretaceous plants display a transition character from the vegetation of the secondary to that of the tertiary periods. Coniferæ and Cycadeæ still flourished as in the preceding oolitic and triassic epochs; but with these fossils, some well-marked leaves of dicotyledonous trees referred to several species of the genus Credneria, had been found in Germany in the Quadersandstein and Pläner-kalk. Still more recently, Dr. Debey of Aix-la-Chapelle has met with a great variety of other leaves of dicotyledonous plants in the cretaceous flora[xvi-B],] of which he enumerates no less than 26 species, some of the leaves being from four to six inches in length, and in a beautiful state of preservation. In the absence of the organs of fructification and of fossil fruits, the number of species may be exaggerated; but we may nevertheless affirm, reasoning from our present data, that in the lower chalk of Aix-la-Chapelle, Dicotyledonous Angiosperms flourished nearly in equal proportions with Gymnosperms; a fact of great significance, as some geologists had wished to connect the rarity of dicotyledonous trees with a peculiarity in the state of the atmosphere in the earlier ages of the planet, imagining that a denser air and noxious gases, especially carbonic acid in excess, were adverse to the prevalence, not only of the quick-breathing classes of animals, (mammalia and birds,) but to a flora like that now existing, while it favoured the predominance of reptile life, and a cryptogamic and gymnospermous flora. The co-existence, therefore, of dicotyledonous angiosperms in abundance with Cycads and Coniferæ, and with a rich reptilian fauna comprising the Iguanodon, Ichthyosaurus, Pliosaurus, and Pterodactyl, in the lower cretaceous series tends, like the oolitic mammalia of Stonesfield and Stuttgart, and the triassic birds of Connecticut, to dispel the idea of a meteorological state of things in the secondary periods widely distinct from that now prevailing.
General remarks.—In the preliminary chapters of "The Principles of Geology," in the first and subsequent editions, I have considered the question, how far the changes of the earth's crust in past times confirm or invalidate the popular hypothesis of a gradual improvement in the habitable condition of the planet, accompanied by a contemporaneous development and progression in organic life. It had long been a favourite theory, that in the earlier ages to which we can carry back our geological researches, the earth was shaken by more frequent and terrible earthquakes than now, and that there was no certainty nor stability in the order of the natural world. A few sea-weeds and zoophytes, or plants and animals of the simplest organization, were alone capable of existing in a state of things so unfixed and unstable. But in proportion as the conditions of existence improved, and great convulsions and catastrophes became rarer and more partial, flowering plants were added to the cryptogamic class, and by the introduction of more and more perfect species, a varied and complex flora was at last established. In like manner, in the animal kingdom, the zoophyte, the brachiopod, the cephalopod, the fish, the reptile, the bird, and the warm-blooded quadruped made their entrance into the earth, one after the other, until finally, after the close of the tertiary period, came the quadrumanous mammalia, most nearly resembling man in outward form and internal structure, and followed soon afterwards, if not accompanied at first, by the human race itself.