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Both in Europe and America the land and freshwater shells accompanying the extinct pachyderms are of living species, and I could detect no shell in the Natchez loam so foreign to the basin of the Mississippi as is the Cyrena fluminalis to the rivers of modern Europe.
No naturalist would for a moment suppose that the extermination of the Cyrena fluminalis throughout the whole of Europe a species which co-existed with our race in the valley of the Somme, and which was very abundant in the waters of the Thames at the time when the elephant, rhinoceros, and hippopotamus flourished on its banks was accelerated by human agency.
During the levelling up of these old channels, a flood sometimes cuts into and partially removes portions of the previously stratified matter, causing those repeated signs of furrowing and filling up of cavities, those memorials of doing and undoing, of which the tool-bearing sands and gravels of Abbeville and Amiens afford such reiterated illustrations, and of which a parallel is furnished by the ancient alluvium of the Thames valley, where similar bones of extinct mammalia and shells, including Cyrena fluminalis, are found.
When we ascend the valley of the Somme, from Abbeville to Amiens, a distance of about 25 miles, we observe a repetition of all the same alluvial phenomena which we have seen exhibited at Menchecourt and its neighbourhood, with the single exception of the absence of marine shells and of Cyrena fluminalis. In both the upper and lower gravels, as Dr.
I have seen an example of this at Ilford, where the Thames, or a tributary stream, has at some former period cut through sands containing Cyrena fluminalis, and again filled up the channel with argillaceous matter, evidently derived from the waste of the Tertiary London Clay.
The relation of the glacial period to alluvial deposits, such as that of Gray's Thurrock, where the Cyrena fluminalis, Unio littoralis, and the hippopotamus seem rather to imply a warmer climate, has been a matter of long and animated discussion.
Interior of right valve of a small specimen, from Shacklewell, London. d. LIVING: Tellina fluminalis, O.F. Muller : 1774. Venus fluminalis Euphratis, Chemnitz : 1782. Cyclas Euphratica, Lam. : 1806. Cyrena cor, Lam. Cyrena Cashmiriensis, Desh. : Corbicuia fluminalis, Muhlfeldt. : 1811. FOSSIL: Cyrena trigonula, S. Woodward : 1834. Cyrena Gemmellarii, Philippi : 1836.
Such old alluvial deposits now capping the cliffs of Kent seem to have been the river-beds of tributaries of the Thames before the sea encroached to its present position and widened its estuary. On following up one of these freshwater deposits westward of the Reculvers, Mr. Prestwich found in it, at Chislet, near Grove Ferry, the Cyrena fluminalis among other shells.
It is probable that causes more general and powerful than the agency of Man, alterations in climate, variations in the range of many species of animals, vertebrate and invertebrate, and of plants, geographical changes in the height, depth, and extent of land and sea, some or all of these combined, have given rise in a vast series of years to the annihilation, not only of many large mammalia, but to the disappearance of the Cyrena fluminalis, once common in the rivers of Europe, and to the different range or relative abundance of other shells which we find in the European drifts.
Marine Shells in same. Cyrena fluminalis. Mammalia. Entire Skeleton of Rhinoceros. Flint Implements, why found low down in Fluviatile Deposits. Rivers shifting their Channels. Relative Ages of higher and lower-level Gravels. Section of Alluvium of St. Acheul. Two Species of Elephant and Hippopotamus coexisting with Man in France. Absence of Human Bones in tool-bearing Alluvium, how explained.
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