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Physa columnaris, Desh. Melanopsis buccinoidea, Ferr.; recent. The univalve shells most characteristic of fresh-water deposits are, Planorbis, Limnaea, and Paludina. Neritina globulus, Def. Nerita granulosa, Desh.
To some it may appear enigmatical how such fragile objects could have escaped annihilation in a river-bed, when flint tools and much gravel were shoved along the bottom; but I have seen the dredging instrument employed in the Thames, above and below London Bridge, to deepen the river, and worked by steam power, scoop up gravel and sand from the bottom, and then pour the contents pell-mell into the boat, and still many specimens of Limnaea, Planorbis, Paludina, Cyclas, and other shells might be taken out uninjured from the gravel.
Now each egg has a sort of lid, which presently opens and lets out a minute, hairy creature who swims away in search of a particular kind of water-snail the kind called by naturalists Limnaea truncatula. If he finds a snail, he bores his way into its flesh and soon begins to grow and wax fat.
Gilbert found a new species of sleeping lizard, with four lighter stripes on the dark brown ground along the back, and with dark spots on the sides. Mr. Roper shot some ducks, and I found a species of Ancylus; besides the species of Limnaea and Paludina, which we had previously met with. Dec. 28.
The slug, called Succinea, is not strictly aquatic, but lives in damp places, and may be seen in full activity far from rivers, in meadows where the grass is wet with rain or dew; but shells of the genera Limnaea, Planorbis, Paludina, Cyclas, and others, requiring to be constantly in the water, are extremely exceptional in the loess, occurring only at the bottom of the deposit where it begins to alternate with ancient river-gravel on which it usually reposes.
The great dirt-bed or vegetable soil containing the roots and stools of Cycadeae, which I shall presently describe, underlies these marls, and rests upon the lowest fresh-water limestone, a rock about eight feet thick, containing Cyclas, Valvata, and Limnaea, of the same species as those of the uppermost part of the Lower Purbeck, or above the dirt-bed.
Of the former kinds, I found a Limnaea in great numbers in a lake, into which, the inhabitants assured me that the sea enters once a year, and sometimes oftener, and makes the water quite salt. I have no doubt many interesting facts, in relation to marine and fresh water animals, might be observed in this chain of lagoons, which skirt the coast of Brazil.
Limnaea, and two species of Melania, were found in it; the one species, with a long sharp spire, had been found in a reedy brook, at the upper Burdekin. Limmen Bight river was not half a mile from our camp; and I now hoped that we should soon be out of the system of salt-water creeks joining it from the southward. We had left the stiff grasses of the coast, and the pasture was fast improving.
Limnaea, similar to those of Europe, creep along the surface of the water; small Planorbis live on the water-plants, to which also adhere Ancylus; and Paludina, Cyclas, and Unio, furrow its muddy bottom. The latitude of this spot was 21 degrees 23 minutes S.
Calvert collected a great number of Limnaea in the water-holes: its shell is more compact than those we have before seen, and has a slight yellow line, marking probably the opening at a younger age. Several insects of the genera Mantis and Truxalis were taken, but did not appear different from those we had previously collected. Jan. 31.
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