Part 7

Limnæa (Amphipeplea) glutinosa.--Very local. Somewhat like Ph. fontinalis, but larger and more thin. In young specimens the mantle covers the shell, and in adults the animal is not wholly contained in the shell.

Limnæa (sub-genus Radix) involuta.--Only found in one Irish tarn. Whorls envelop the spire. Very thin, pale amber.

Limnæa (Radix) peregra.--The most common and variable of all our freshwater shells. Spire pointed. Somewhat amphibious. Found practically over the whole of the Eastern Hemisphere.

Limnæa (Radix) auricularia.--Mouth very large, with outer lip widely reflected. Very common and fine in the Thames. Spire very short, apex sharp.

Limnæa (sub-genus Limnophysa) stagnalis.--The largest of the genus. Common, except in Wales. Shell greyish, spire long and tapering to a point; 12210 teeth on its lingual ribbon. See the illustration on p. 57, which also shows above two specimens of Paludina contecta, one being covered (as freshwater shells often are) by a vegetable growth, which obscures the marking.

Limnæa (Limnophysa) palustris.--Shell tapering, somewhat solid, brown, much smaller than stagnalis. Common in slow or stagnant water. Some varieties much darker than the type.

Limnæa (Limnophysa) truncatula.--Like the last in shape, but much smaller, and with a deeper suture. Common, and fond of being out of the water. A parasite of this mollusc causes "fluke" in sheep which have taken it in by drinking or by eating grass by the side of ponds and ditches.

Limnæa (Omphiscola) glabra.--Also amphibious. About the same size as truncatula. Local. Inner lip rather thick and reflected on the base of the penultimate whorl.

Limnæa (Ancylus) fluviatilis.--"Freshwater limpet." Shell, rather limpet-like, with a hooked apex (whence its generic name), adheres to stones or piles in running water. Common. I once dredged a large water-beetle with three of these shells adhering to its wing-cases; thus it would be transported to fresh habitats.

Limnæa (Acroloxus) lacustris.--Like the former but more local, and preferring sluggish or still waters. Shell more oblong, thinner, and apex twisted to the left instead of to the right as in fluviatilis.

The sub-order Pectinibranchiata (comb-like gill) contains the genera Neritina, Paludina, and Valvata, in all of which there are two tentacles with eye at the base, and an operculum to the shell.

Neritina fluviatilis.--Solid, glossy, chequered brown, white, and purple (but also a lemon-coloured variety). Operculum semi-lunar, orange, with a projection which serves as a lock to keep the operculum in position. Not rare in England; on stones in running water. See illustration below, which also shows above L. (Ancylus) fluviatilis.

Paludina (Vivipara) contecta.--Shell dark green with darker bands. Conical. Suture very deep. Operculum horny. Viviparous. Local.

Paludina (Vivipara) vivipara.--More common than contecta. Shell more oval, not so glossy, light greenish yellow, suture not so deep, no umbilicus, apex blunt.

Paludina (Bythinia) tentaculata.--(The eyes in this genus are not on foot-stalks; the operculum is shelly instead of horny). Common in slow water and ditches. Shell semi-transparent, yellowish, mouth oval, angulated above. Operculum made of plates rising one above another formed at different stages of growth.

Paludina (Bythinia) leachii.--Much smaller and less common than the last. Distinct umbilicus; mouth almost circular.

Paludina (Paludestrina) ventrosa.--A brackish-water shell, swarming where found, e.g. from Erith to Gravesend, and in East Anglia. Shell small, thin, semi-transparent.

Paludina (Paludestrina) jenkinsi.--A larger shell, not confined to brackish water and spreading very rapidly. Swarms where found. A variety has a marked keel which sometimes bears bunches of spines at equal distances.

Paludina (Paludestrina) stagnalis.--Larger and with more whorls. Not so common.

Paludina (Pseudamnicola) anatina.--Small, sub-conical, deep suture. Found in brackish water, and apparently identical with Hydrobia or Paludestrina similis, which I used to find by the Thames, where it is now apparently extinct.

Valvata piscinalis.--Globular, suture very deep, circular mouth, operculum concentrically spiral. In ponds and slow water. Shell yellowish, but commonly covered with conferva.

Valvata cristata.--Much smaller; shell disk-shaped. Frequents the roots of flags. Shell striated and more or less ridged, but the name cristata refers to the plume-like appearance of its breathing apparatus.

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We now come to the bivalve shells with leaf-like gills. The Unionidæ contain two genera, Unio and Anodonta, commonly called freshwater mussels.

Unio tumidus.--Shell ovate, very solid, dark brown; common. See accompanying illustration, which shows the fringed branchial siphon which draws in food-bearing water, and the smaller anal siphon by which it gets rid of undigested matter.

Unio pictorum.--More oblong and thinner shell, yellowish, girdled with brown in the lines of growth. Common. The specific name recalls that gold and silver paint used to be sold in these shells (or marine mussels) for illuminating work. It is said to produce 220,000 eggs in the three summer months.

Unio (margaritana) margaritifer.--Shell solid and black, beaks always eroded. Mainly found in mountain streams. Its pearls are few and poor compared with those of marine shells; but they attracted the notice of Cæsar and so hastened the conquest (and development) of Britain.

Anodonta cygnea.--(In this genus the hinge is toothless, whence its generic name. The specific names cygnea and anatina mean "swan" and "duck," in reference to their comparative size). This is the largest of our freshwater shells, reaching even 9 inches in breadth by 4-1/2 in length. Common in ponds and slow water. Sometimes the shells are yellowish green with rays of the same colour.