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valves, through the vast series of deposits which extends from the Devonian rocks to the present ocean -similar forms, similar accompaniments, and similar exclusions of other forms, such as Brachiopoda, Monomyaria, &c. So that the law of segregation (if we may use this term) which separates the few freshwater from the many marine races, operates now as it did operate in the Palæozoic ages, and similar races are subjected to its influence. During all this time Unionidæ have maintained their characteristic structure and associations. Taking next the Gasteropoda we remark in living nature two groups, one separating air from water by means of gills like fishes, the other breathing air by lungs. Of the former, the living examples cluster round the genera, Paludina, Valvata, Ancylus, Neritina, the former are represented by Planorbis, Limnæa, Physa, Melania. In a fossil state these forms are recognized at several stages, but not at so early a period as the Dimyarians. They are not known to occur below the Purbeck Strata.

Cænozoic...Paludina, Neritina, Planorbis, Physa, Limnæa, Me

lania, &c. several zones in the Fluvio-marines

of the Isle of Wight, Wealden ...Paludina, Melania, Melanopsis. Purbeck ... Paludina, Valvata, Melania, Physa, Limnæa,

Planorbis.

The freshwater Mollusca altogether appear of later origin than the marine tribes, and the Gaste ropoda appear to be of later origin than the Dimyaria, in fresh waters as well as in the sea. If we compare the fossil and living Paludinæ, Physæ, Limnææ and Planorbes, we are struck by the great resemblance in each case; Physa in each case has the sinistral spire, Planorbis is discoidally depressed, Paludina rises in tumid whorls, Limnæa extends from a large aperture to an acute point. Here again is the evidence of long duration of even slight peculiarities, persistence of type, and restraint of variation.

If in either of these cases—the Unionidæ-the Paludinadæ-- the Limnæadæ - Planorbes -- Physæ, &c.,—the modern forms are derived from the ancient, we have the full measure of the whole variationthe differentials of change are all integrated by time, and we behold the sum-how little! But if not so, if the modern and ancient species have sprung from different branches of a stem still older than either, how much stronger, if possible, is this decisive testimony against the doctrine of indefinite change through time and circumstance! Circumstances have varied, ages have passed away, and yet every generic group exhibits at every step the same essential characters, and many of the little peculiarities, such as

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eroded beaks, plications on the surface, reflexions of the lip, carinations of the whorls, which cannot be consistent with accumulated tendencies to change.

If in the same spirit we look at the few Insects, Crustaceans and Reptilia, the only other freshwater groups which occur at several stages in the series of strata, very similar results appear, much resemblance in the components of the several groups from whatever age we take the examples, and much adherence to generic, or family type. The series of freshwater Reptiles considered in this respect offers the largest variations, but the structure of some of the earlier forms, even in regard to the limbs, is too uncertain to allow of a satisfactory view of their history. True Crocodiles with vertebræ concave in front, occur above the Chalk"; Crocodilians with biconcave vertebræ-an indication of aquatic, but not necessarily of marine life—are frequent in the Oolites and Lias?, and are found in the Trias”, with a Lacertian fossil". Batrachians with deciduous gills are mostly of Tertiary date”, but allies of the Menopoma and Triton appear in the Coal Strata, and in Strata above them. Freshwater Chelonians are recognized in the Strata of Purbeck and in Tertiary deposits.

1 Croc. Harsingsiæ.

3 Teleosaurus and Steneosaurus, 3 The Stagonolepis of Elgin. 4 Rhynchosaurus of Shropshire.

5 In the Paper Coal of the Rhine.
& Archegosaurus-Labyrinthodon.

TERRESTRIAL LIFE.

The information afforded by freshwater deposits, and the effects of freshwater currents flowing into the sea, is strongly reflected in the history of the ancient land. As the stream flows down from its parent mountains, gathering air and penetrated by light, it soon begins to swarm with infusoria, feeding among the immersed and marginal vegetation; Planorbes, Limnæadæ, Cycladæ, appear in its sparkling or quiescent water; Gammarinæ and Daphniæ, Insects and Argyronetæ, furnish delicate diet to the watchful Trout; and thus race follows race, till finally Unionidæ prevail and occupy the bed of the current as it slowly winds toward the estuary, which, itself almost deserted, divides the fluviatile from the oceanic worlds of life.

When in the Coal-formation we find an abundance of Unionidæ, often buried in colonies as they lived, in layers of fine sediment, alternating with Coal Strata, accumulated towards the margin of the sea, we may easily connect in imagination this marshy savannah with rivers descending from the interior, and suppose on their banks many races of animals whose remains, if rare, may nevertheless occur to diligent search on favourable occasions.

By such scrupulous attention to the contents of Ironstone nodules at Coalbrook Dale Mr Anstice discovered two species of Coleoptera (Curculionidae), and a Neuropterous Insect (Corydalis)'. But the most remarkable illustration of the utility of this kind of hopeful search is afforded by the discovery of a landshell, allied to if not identical with Pupa, in the interior of a fossil tree (Sigillaria), in the Coal-formation of Nova Scotia, by Sir C. Lyell and Dr Dawson. Remains of Land Reptiles (Dendrerpeton and Hylonomus), and a Chilognathous Myriapod (Xylobius). An airbreathing Gasteropod of a modern genus, airbreathing Reptiles, Insects of two recognized orders, and a Myriapod, these suggest and indeed imply the existence of many more forms of terrestrial animals so as to constitute a Fauna of the Carboniferous age. Precarboniferous we might perhaps say, for plants of the family (Lepidodendra) with which these insects and reptiles and shells are associated occur in earlier strata, being first noticed in the uppermost bands of the Silurian system.

Terrestrial plants are scattered at intervals through most of the Marine Strata, above the Silurian Rocks, thus indicating the force and frequency of affluents from the land. They were collected in considerable quantity in estuarine and lacustrine deposits of the Carboniferous and Oolitic eras, and in lagoons of salt water, as at Stonesfield, having been,

Prestwich, On Coolbrook Dale Coalfield, Geol. Trans.

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