of the Old Red strata, in which there are no Mollusca, are species of Auchenaspis, Onchus (2 species), Pteraspis, Cephalaspis, and Plectrodus. These also in the main indicate a change of conditions, which were, I believe, of a geographical kind.
The Eurypteridae and Pterygoti in England almost entirely belong to the passage-beds ; and one, Eurypterus Symondsii, is only found in the lower Old Red strata.
The circumstances which marked the passage of the uppermost Silurian rocks into Old Red Sandstone seem to me to have been the following : — First, a shallowing of the sea, followed by a gradual alteration in the physical geography of the district, so that the area became changed into a series of mingled fresh and brackish lagoons, which finally, by continued terrestrial changes were converted into a great freshwater lake, or, if we take the whole of Britain and lands beyond, into a series of lakes ; and the occurrence of a very few genera or even species of fish and Crustacea, common both to the fresh and the brackish or even salt waters, does not prove that the Old Red Sandstone is truly marine. At the present day animals that are commonly supposed to be essentially marine, are occasionally found inhabiting fresh water. Thus I am informed by Mr. Murray, of the Geological Survey of Canada, that in the inland fresh lakes of Newfoundland seals are common. They breed there freely, and never visit the sea. The same is the case in Lake Baikal, in Central Asia ; and though these facts bear but slightly on my present subject, seals being air-breathing Mammalia, yet in some of the lakes of Sweden it is said that marine Crustacea are found. This may be accounted for in the same way that I now attempt to account for these peculiarities in the Old Red Sandstone strata. These Swedish lakes were submerged during the Glacial period ; and being deep basins (scooped out it matters not by what process), while the land was emerging, and after its final emergence, the salt water of the lakes freshened so slowly, that some of the creatures inhabiting it had time by degrees to adjust themselves to new and abnormal conditions*.
Again, we may suppose a set of circumstances such as the following : — If by changes of physical geography of a continental kind a portion of the Silurian sea got isolated from the main ocean, more or less like the Caspian and the Black Sea, then the ordinary marine conditions of the " passage beds," accompanied by some of the life of the period, might be maintained for what, in common language, seems to us a long time. The Black Sea was once united to the Caspian, the two together forming one great brackish lake. The Black Sea is now steadily freshening ; and it is easy to conceive that by the closing of the Bosphorus (a comparatively small geographical change) it might be again converted into a fresh lake. At present a great body of salt water is constantly being poured out through the Bosphorus, and its place taken by the fresh water of rivers. At pre-
- For much important information on this subject, see Annals and Mag. of
Nat. Hist., 3rd series, vol. i. 1858, p. 50, " On the Occurrence of Marine Animal Forms in Fresh Water," by Dr. E. Ton Martens. Translated by Mr. W. S. Dallas.
