11. Chelonida (Turtles), III. Ophidia (Snakes), IV. Lacertilia (Lizards), and v. Crocodilia (Crocodiles), are both recent and fossil; and eight, viz. vI. Deinosauria, VII. Thecodontia, VIII. Pterosauria, Ix. Anomodontia, x. Sauropterygia, x1. Ichthyopterygia, XII. Labyrinthodontia, XIII. Ganocephala are only found in a fossil state. Among the marine tribes are some of the Chelonida, all the fossil Ichthyopterygia, Sauropterygia, some of the Crocodilia and Lacertilia, possibly the Thecodontia1, omitting the Chelonida. They stand in the following order of time:

Ichthyosaurus.. Lias and upwards to Chalk. Plesiosaurus Bone Bed, Aust, and upwards to Chalk. Stagonolepis ...... Triassic Sandstone of Elgin. Thecodontosaurus. Conglomerate, Bristol.

It will be seen hereafter that Terrestrial and Freshwater Reptilia appear to be of higher antiquity than the marine tribes yet discovered. Remains of Chelonida appear to be about of the same antiquity as the marine Saurians; they occur in the Red Sandstone of Lochmaben in Dumfriesshire, with various undetermined Ichnites.

Cetacea appear to be totally absent from any of the deposits older than the Tertiary Strata, a circumstance more remarkable since the discovery of terrestrial Mammalia in the Oolitic Strata of Purbeck and Stonesfield, and the Bone-bed of Würtemberg. The Crag of Suffolk (Pleiocene) is the oldest tertiary deposit in our Islands containing Cetacea. In no district of Europe do they reach downward even to the earliest series of Tertiary Strata. Above this D'Orbigny counts four genera in the Parisian, seven in the Falunian, five in the sub-Apennine strata -fifteen, the maximum, in the modern seas. In a general point of view, the Cetacea, Great Reptiles, and Great Fishes,

may be regarded as successively the dominant races of the Sea, the Cetacea taking up the functions which had been exercised by the Enaliosaurians. The earlier races of Cestraciont Fishes, with their crushing teeth, may be thought very well suited to such food as the shelly Mollusca, the stronglywalled Encrinites, and the cuirassed Crustacea of the early periods might furnish; and when their

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 lacune. 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.

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

Cænozoic Period..

Alternating freshwater and marine
Strata of the Isle of Wight.

Mesozoic Period

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

Palæozoic 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. Unionidæ and Cyclade are the prevailing family of Dimyarian Mollusks; Paludinade 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,