influence declined in the Tertiary Seas, the skates and rays, furnished with a suitable pavement of teeth, may be supposed to have assumed some of their duties.

The stratified rocks on which we base the general scale of elapsed time being marine, and the occurrence of freshwater deposits among such being necessarily limited, it is not surprising that our history of freshwater life is marked by many lacunæ. Rather may we be surprised that the nature of the movements of the earth in ancient times was such that marine deposits were covered by lakes, or estuaries, at several epochs, and sometimes for long periods of time. In the following brief catalogue, suited to the British Isles, we find what may be termed Freshwater zones in all the great systems of Strata.

Cenozoic Period..

Mesozoic Period

Postglacial shelly marls, &c,
Preglacial shelly marls, &c.

Alternating freshwater and marine
Strata of the Isle of Wight.

The deposits of the Weald of Sussex.
The deposits of Whitby and Scarborough.

Paleozoic Period ... The Coal-formation.

The Upper part of the Old Red Sandstone of Ireland.

And besides, there are several cases of the intermixture of land-plants and insects and sea-shells, only to be explained by the flowing of currents from the land, as at Stonesfield, Westbury Cliff, &c.

The forms of life in the Fresh waters of the Earth differ from those in the Sea in all cases; but the difference can seldom be traced to any physiological necessity, arising from the difference of the fluids. Amorphozoa, Zoophyta, Mollusca, Annelida, Crustacea, Fishes, Reptiles and Mammalia, occur in both, under shapes not indeed identical, but fashioned on the same general models. The salmon migrates from one water to the other, and experiments appear to shew that by long continuance of favourable circumstances other fishes and some Mollusca might be made to exchange elements, or to subsist for a time in brackish water, a fluid of intermediate character. But, on a large scale, the provisions of nature keep

separate the denizens of lakes and rivers from those of the lagoons, bays, and currents of the sea.

This fact, which is observed in all countries, is connected with another of much significance, the comparative fewness of the freshwater races, and their great affinity over large tracts of the earth. Unionida and Cycladæ are the prevailing family of Dimyarian Mollusks; Paludinada and Limnæadæ, reinforced by Ancylus, Melania, Neritina, &c. the most frequent of Gasteropoda. How these freshwater genera have been so widely diffused as in fact we find them; how some particular species have become common to so many rivers and lakes, are questions of much interest. For as neither the animals nor their ova could pass through the salt water, and retain vitality, we must ascribe to changes in physical geography and the accidents of mixed occupation of a country, the transference from one river to another, of the germs of life; or regard the existing fresh waters, now unconnected, as formerly somehow connected; or suppose a creation of the same species or the same genera at many separate points. Rejecting this last, we may venture to prefer, as a general explanation, the transfer of the germs of life by natural events, specially by birds carrying spawn from one river to another, even as now we are transferring by experimental means the Crawfish,

the Trout, and other valued creatures, to rivers which have never yet been visited by them.

The results arrived at by tracing some of the successive groups of freshwater life, from the earliest date to their eventual distribution, are very instructive in regard to what may, with reference to human ideas, be termed the Theory of Creation. Taking first the molluscous classes and remembering that Brachiopoda and Monomyaria are absent from fresh waters, we may fix our attention on the Dimyarian family of Unionida and Cycladæ. They occur fossil according to the following scale:

Cenozoic-Unio, and Cyclas, not very plentiful, with Gasteropoda and land-plants.

Mesozoic Wealden....... Unio and Cyclas, with Cyprides and Gasteropoda and landplants.

Purbeck ......... Unio and Cyclas, with Cyprides

and Gasteropoda and land-plants (Cycadaceae).

Yorkshire Oolitic. Unio not common, with land

plants (Cycadaceæ, &c.)

Paleozoic-Coal-formations. Unio, several species, common,

with land-plants, Lepidoden

dron, &c. Cyprides, &c.

Upper Old Red. Unio, in one locality, with Ferns.

Here we have the remarkable fact of decided affinity in the successive groups of freshwater bi

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.

Cenozoic...Paludina, Neritina, Planorbis, Physa, Limnæa, Melania, &c. several zones in the Fluvio-marines of the Isle of Wight.

Wealden...Paludina, Melania, Melanopsis.

Purbeck ...Paludina, Valvata, Melania, Physa, Limnæa, Planorbis,