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that at least a more copious preservation of organic remains might be expected, for the oolitic strata are mostly depositions which have taken place without disturbance.
And so it proves. The Placoids and Ganoids hitherto predominating in the ocean almost without a foe, now found overwhelming enemies in the true sea-lizards, or Enaliosaurians, especially the Ichthyosaura and PlesioThe head is like a lizard or a crocodile, the vertebral column fish-like, and, as Gegenbauer has shown, the extremities also recall the simpler classes of sharks. Their coprolites likewise allow us to infer with full certainty, a very peculiar construction of the middle portion of the intestinal canal. They possessed a spiral intestine like that of the sharks and their congeners. These animals are therefore noteworthy, not only on account of their striking external appearance and the part they play in nature's household, but, like the Labyrinthodonton, as mongrel and connecting forms of reptiles and of fish.
In addition to these animals, we must distinguish among the marine fauna the Ammonites, which now appear in vast masses, and the Nautili, the second chief form of the ancient Cephalopods, the study of which has recently promised to contribute essentially to the decision of the most important points in our science. In combination with them, the Belemnites abound with their multitudinous species, originating in the Trias. They are proved to be the predecessors of dibranchiate Cephalopoda, which now predominate. On the chalk plains of Eichstadt and Solnhofen, belonging to the White Jura, are also preserved impressions, resembling
drawings, of Medusa, which show that even at that time this class had reached the state in which it still exists.
The terrestrial fauna of the Jurassic period is likewise enriched by new forms and groups. We find the first true crocodiles, tortoises, and the most remarkable variation of the Sauroid type, the winged lizard or Pterodactyl. It is evident from their well-preserved skeletons that the wing membrane was stretched, as in the bat, between the posterior and anterior extremities. Behind, it extended to the foot, while in front, it obtained a corresponding addition by the elongation of the little finger. A first and only bird has likewise been found in the well-known resting-places of the Pterodactyls, in the lithographic slates of Solnhofen in Bavaria (Archæopterix lithographica). The most remarkable peculiarity of this bird, recognizable by the most minute impression of its feathers, is the long tail, bordered by two rows of rigid feathers. The head is unfortunately crushed beyond recognition. The inferior order of Mammals already mentioned, the Marsupials, were also present, as is shown by the enclosures of the middle Oolite of England and the upper Oolite of the Purbeck strata.
The ornithic animals of the chalk, are more remarkable intermediate forms than the Archæopteryx, and these by their hour-glass-shaped vertebrate bodies are directly connected with the sea-lizards of the Jura, and also possess teeth; this may, however, be the case with the Archæopteryx also. We shall return later to these creatures, which fill up a void hitherto painfully sensible. During this new period the Ammonites were most abundant, and then became extinct, after going through a stage of degenerate forms which may be observed in the
Turrilites, Scaphites, Baculites, and others are considered. The prime of the great sea-lizards is also past, but the marshes of the Wealden period harboured new forms of colossal land-lizards. The long-tailed cray-fishes are joined by the true crabs, the most highly developed forms of the class. In the Oolite and Chalk also occur the chief of the sea-urchin-like Echinoderms. As yet we have not mentioned the class of Echinodermata, in order that we might here point out in conjunction several of the more important phases of their geological occurrence. Desor,* a distinguished judge of this class, has lately examined how in this large group of Echina the progress of organization is gradually manifested, on which occasion he was induced to make some general reflections on the principle of progression, as applied to the Echinoderms, probably known to all our readers in their representatives the star-fish and sea-urchins. If articulate, as well as vertebrate, animals attain a higher grade of development by the differentiation of the consecutive segments of the body, the superior unity, and therewith higher perfection, of the Echinoderm's body is evinced when the spines, or so-called antimera, give way to the unity of the whole.
The more distinct these elements are, that is to say, the more independent they remain, the lower is, not only the articulate animal, but also the Echinoderm. Accordingly, the star-fish, and to some extent the feather-stars, stone-lilies, or crinoids, occupy the lowest rank. But here, unluckily, paleontological tradition likewise abandons us. Only so much is certain, that in the older fossiliferous strata both divisions are abundantly repre
* Bulletin de la Société des Sciences Naturelles de Neufchâtel, IX. 2.
sented. A highly remarkable and important intermediate form is also known, found in the upper Silurian strata of Dudley (Eucladia Johnsoni), the more important as but few transitional forms between one order and another have been hitherto discovered. The relation of the star-fish to the sea-urchins is still indistinct. On the other hand, the bridge from the stone-lilies to the sea-urchins is tolerably apparent. The true Crinoids are sessile, and with them are connected, in the carboniferous formation, the no longer sessile Cystoids and Blastoids, with which are associated the Tessellæ, more resembling the sea-urchins. Now the Dyas and Trias are still poor in true Echina; the Jura, on the contrary, very rich; and in this great period the extraordinarily heterogeneous transformations of the Echina are slowly accomplished, and may be traced, step by step, from the Lias, the earliest oolitic formation, to the coral limestone. At first the Cidaridæ predominate; they are joined in the Oolite by the Echinoconidæ and Cassidulidæ. In the upper layers of the Jura, the sharper separation of the species becomes characteristic.
Desor shows how this development, accompanied by temporary quiescence, is connected with the nature of the sea-bottom at the time. "The law of progress," he says, "is displayed in the circumstance that it is the lowest of the Echinæ, the Regularæ and Endocyclicæ, which primarily appear, first in the form of the Tessellæ, then as Cidarida; while the most perfect Spatangæ, with the most distinctly marked bilateral form, make their appearance last of all. Between these extremes we find a host of genera and species distinguished from one another by mere shades, so that of two allied genera it is
often difficult, nay, impossible, to state which is the more perfect. Progression is only to be shown collectively; in the concrete case it can rarely be demonstrated."
The Échinæ still predominate in the chalk. Recent discoveries of analogous animals, with soft and flexible persistent integuments, confirm what was theoretically extremely probable, that from them proceeded the highest existing order of the Holothuria or Sea-cucumbers; and thus the division of Echinoderms conforms to the universal experience of the ascent from the lower and undifferentiated to the higher forms.
With the Tertiary period dawns the state of things now existing. Palms and arboraceous plants characterize the vegetation. The animal world has likewise remained essentially the same from the earliest sections of the Tertiary period until now, as we shall more elaborately set forth in the chapter on Geographical Distribution. In the most ancient formations the Fishes, in the middle the Reptiles, were conspicuous in the world. of life as the representatives of the highest development; now when the continents, not indeed without sundry local oscillations, are approximating to their present configuration, the impress of the Mammalia becomes predominant. Under the influence of elevations and depressions, of several glacial periods, and the more sharply defined limits of the climatic zones, frequent displacements occurred in the vegetal and animal world, accompanied by differentiation and further development. As we have already mentioned, the course of our inquiries will bring us back to this subject.
At the time when geologists believed in the rigid partition of the earth's periods of development and the sharply