The freshwater Mollusca altogether appear of later origin than the marine tribes, and the Gasteropoda 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 Unionida-the Paludinada-the Limnæada-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 R. L. I 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 Lias2, 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 The Stagonolepis of Elgin. 4 Rhynchosaurus of Shropshire. 5 In the Paper Coal of the Rhine. 2 Teleosaurus and Steneosaurus. • 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 (Curculionidæ), 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, 1 Prestwich, On Codbrook Dale Coalfield, Geol. Trans. perhaps, blown into these latter situations with Neuropterous and Coleopterous Insects, among the latter, Buprestidæ and Curculionidæ. In the Paleozoic. ages are no Cycadaceae; in the Neozoic no Lepidodendra; Ferns abound in several of the deposits in and above the Upper Devonian Strata. In the lowest beds of the Lias and passage-beds from the Trias, Insects have been collected at the Cliffs of Aust, Westbury and Wainlode, and at several other places in the Vale of the Severn, probably blown into shallow salt water, a common circumstance on the coasts. Others occur more abundantly in the Vale of Wardour and Purbeck'. The census of the fossil orders of Insects runs thus: Cenozoic...Coleoptera (Copris, Donacia, Harpalus,) in Pleistocene beds, at Mundsley, Norfolk. (In France most of the Orders of Insects are found in freshwater beds at Aix in Provence). Mesozoic...Purbeck beds, Coleoptera, Neuroptera, Orthoptera, Homoptera, Diptera. Oolite of Stonesfield, Coleoptera, Neuroptera. Paleozoic...Coal-formation, Coleoptera, Neuroptera. Terrestrial Saurian Reptiles acquired extraordinary magnitude in the Oolitic period, and exhibit as 1 Brodie, On Fossil Insects. Westwood has determined many of the orders and genera. |