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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.
In the last-named locality a Cytherea inhabited the mud around their roots. At the Three Islets several new species of Melampus, a Nerita and a Cyrena lived in a like habitat, and at Port Essington Cerithium kieneri, was found in the same situation. The fine Cyrena cyrenoides lives among the roots of mangroves in the Louisiade Archipelago.
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.
They overlie the uppermost of an extensive series of Eocene deposits of marine, brackish, and fresh-water formations, which rest on the Chalk and terminate upward in strata corresponding in age to the Paris gypsum, and containing the same extinct genera of quadrupeds, Palaeotherium, Anoplotherium, and others which Cuvier first described. Corbula pisum. Cyrena semistriata. Cerithium elegans.
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.
Buckland found a large pebble to which five full-grown oysters were affixed, in such a manner as to show that they had commenced their first growth upon it, and remained attached to it through life. Cyrena cuneiformis, Sowerby. Natural size. Syn. Cerithium melanoides, Sowerby.
At New Charlton, in the suburbs of Woolwich, Mr. de la Condamine discovered in 1849, and pointed out to me, a layer of sand associated with well-rounded flint pebbles in which numerous individuals of the Cyrena tellinella were seen standing endwise with both their valves united, the siphonal extremity of each shell being uppermost, as would happen if the mollusks had died in their natural position.
I have described a bank of sandy mud, in the delta of the Alabama River at Mobile, on the borders of the Gulf of Mexico, where in 1846 I dug out at low tide specimens of living species of Cyrena and of a Gnathodon, which were similarly placed with their shells erect, or in a posture which enables the animal to protrude its siphon upward, and draw in or reject water at pleasure.
Many of the marine shells of the brackish-water beds of the above series, both in the Isle of Wight and Hordwell Cliff, are common to the underlying Barton Clay: and, on the other hand, there are some fresh-water shells, such as Cyrena obovata, which are common to the Bembridge beds, notwithstanding the intervention of the St. Helen's series.
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.
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