_Paludina (Vivipara) vivipara._--More common than contecta. Sh.e.l.l more oval, not so glossy, light greenish yellow, suture not so deep, no umbilicus, apex blunt.

[Ill.u.s.tration: _Neritina_ and _Ancylus_.]

_Paludina (Bythinia) tentaculata._--(The eyes in this genus are not on foot-stalks; the operculum is sh.e.l.ly instead of h.o.r.n.y). Common in slow water and ditches. Sh.e.l.l 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 sh.e.l.l, swarming where found, _e.g._ from Erith to Gravesend, and in East Anglia. Sh.e.l.l small, thin, semi-transparent.

_Paludina (Paludestrina) jenkinsi._--A larger sh.e.l.l, 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 (Pseud.a.m.nicola) 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. Sh.e.l.l yellowish, but commonly covered with conferva.

_Valvata cristata._--Much smaller; sh.e.l.l disk-shaped. Frequents the roots of flags. Sh.e.l.l striated and more or less ridged, but the name _cristata_ refers to the plume-like appearance of its breathing apparatus.

We now come to the bivalve sh.e.l.ls with leaf-like gills. The Unionidae contain two genera, Unio and Anodonta, commonly called freshwater mussels.

_Unio tumidus._--Sh.e.l.l ovate, very solid, dark brown; common. See accompanying ill.u.s.tration, which shows the fringed branchial siphon which draws in food-bearing water, and the smaller a.n.a.l siphon by which it gets rid of undigested matter.

[Ill.u.s.tration: Freshwater mussel breathing and eating.]

_Unio pictorum._--More oblong and thinner sh.e.l.l, 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 sh.e.l.ls (or marine mussels) for illuminating work. It is said to produce 220,000 eggs in the three summer months.

_Unio (margaritana) margaritifer._--Sh.e.l.l solid and black, beaks always eroded. Mainly found in mountain streams. Its pearls are few and poor compared with those of marine sh.e.l.ls; but they attracted the notice of Caesar 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 sh.e.l.ls, reaching even 9 inches in breadth by 4-1/2 in length. Common in ponds and slow water. Sometimes the sh.e.l.ls are yellowish green with rays of the same colour.

_Anodonta anatina._--Doubtful if this is a separate species or only a smaller form. The hinge line is raised instead of being straight, and the posterior side slopes abruptly instead of gradually.

In the next family are two genera, Sphaerium and Pisidium.

_Sphaerium corneum._--Very common. Sh.e.l.l somewhat globular, glossy, opaque, horn-coloured, marked with lighter bands in the line of growth. Usually on the bottom, but can suspend itself by threads of mucus.

_Sphaerium rivicola._--Much larger. Also flatter and more striated.

Yellowish brown or greenish. A whole series of young of different sizes will be found in the animal.

_Sphaerium pallidum._--Local in ca.n.a.ls and ponds. Oblong.

Distinguished also from the previous species by the body being milk-white, and the sh.e.l.l is ashy-grey.

_Sphaerium lacustre._--Local. On the beaks is a calcareous nucleus which distinguishes it. It is thinner than _corneum_, and rounder than _pallidum_.

_Pisidium amnic.u.m._--(Our five _pisidia_ resemble _Sphaerium_, but are much smaller, all but _amnicus_ being minute. Very abundant where found. _P. amnic.u.m_ and _fortinale_ are triangular in shape, _P.

pusillum_ oval, _P. nitidum_ round, and _P. roseum_ or _milium_ oblong; but they are difficult to distinguish on account of their similarity and variation). _P. amnic.u.m_ is nearly twice the size of the others, and this and _fontinale_ may be found in slow rivers, whereas the others prefer stagnant waters.

_Pisidium fontinale._--Smaller and thinner, and with more prominent beaks than _P. amnic.u.m_.

_Pisidium pusillum._--The most common species. Distinguished from the last by being oval and by its beaks being blunter and more central.

_Pisidium nitidum._--Rare. Very glossy and striated.

_Pisidium roseum_ (from the colour of part of its body).--Like _nitidum_, but oblong, with a straight lower margin, and with beaks placed away from the centre.

The last sh.e.l.l to be mentioned could not be mistaken for any other. It belongs to the sub-order _Heteromya_ (_i.e._ with adductor or closing muscles not equal); to the family of _Mytilidae_ (or mussels) and the genus _Dreissensia_ (named after a Dutch conchologist).

_Dreissensia polymorpha_ is a triangular, boat-shaped, bivalve, supposed to have been introduced with Russian timber (as was also probably _Hydrobia Jenkinsi_). It is gregarious, and attaches itself to objects by a byssus like our marine mussels. Sh.e.l.l yellowish-brown with wavy purplish lines, wrinkled in the line of growth. Common in the New River, and has been found in iron water-pipes in Oxford Street.

All our sh.e.l.ls have varieties (many an albino or white form), and the collection and distinguishing of these varieties, which in some species are numerous, adds much to the interest of the collector. In addition there are also the variations in size or markings which can hardly rank as varieties. Inasmuch as none of our sh.e.l.ls are peculiar to our country (which is from the natural history and the geological point of view only a detached portion of the Continent), it may be well to warn young collectors that if they receive sh.e.l.ls from the Continent, mere varieties are there often named as separate species and variations considered as definite varieties. This is especially the case with _Helicella virgata_.

As to the arrangement of sh.e.l.ls in a collection before a regular cabinet is obtained, the tinier sh.e.l.ls may be kept in small gla.s.s tubes with corks (such as used for h.o.m.oeopathic medicines), and the medium sized ones in the trays of common matchboxes, these being arranged in large shallow gla.s.s-covered trays which can be obtained from any cardboard boxmaker at a small cost, and several of these, stored one above the other, form an excellent subst.i.tute for a more costly cabinet. In all cases the name, and the place where the sh.e.l.ls were found, should be written on a small slip of card placed in the tube or tray. It is not well in most cases to fasten the sh.e.l.ls on card, but if this is done gum tragacanth is best. The collection should be kept free from damp and from dust.

HINTS FOR COLLECTING AND PRESERVING Sh.e.l.lS OF MOLLUSCS.

The following notes supply a few general rules as to finding and preserving sh.e.l.ls:--

Of Sh.e.l.l-bearing Molluscs there are three cla.s.ses--Marine, Freshwater, and Land. The first two include Univalves and Bivalves, the last only Univalves.

1. MARINE Sh.e.l.lS may be obtained, 1st, by searching on and under rocks at low water, or on coral reefs, among seaweed attached to them, or floating on the sea, or on a sandy beach. Bivalves may be found by digging in the sand, or mud, on a beach, or at the mouth of a river: their presence is generally indicated by a circular breathing hole in the sand. 2nd. By dredging, by which means only deep-sea sh.e.l.ls can be obtained; but after a storm these may often be found upon the sh.o.r.e, before they have lost their l.u.s.tre.

Limpets, etc., should be detached with a thin blade pa.s.sed quickly under the sh.e.l.l, taking care not to break the edges. Small sh.e.l.ls on and in seaweed, and limpets, etc., adhering to stones will drop off and sink to the bottom in a vessel of cold fresh water.

2. FRESH-WATER Sh.e.l.lS may be obtained in any river, lake, pond, marsh or reservoir. Univalves, chiefly on the banks, on reeds and plants growing near the hedges, and on the under surface, leaves, and stems of aquatic plants. Bivalves generally at the bottom, among stones, or buried in the sand, or among the roots of aquatic plants.

3. LAND Sh.e.l.lS.--These resemble, more or less, in their habits the garden snail, though varying greatly in character, size, and colour.

They mostly abound in a chalk or limestone district, and in moist and wooded situations. Some species inhabit low and damp spots, roots of trees, hollows and crevices of rocks and walls; some lie under stones or pieces of wood, or in the earth; others climb shrubs, and in tropical climates even lofty trees. Their haunts vary according to the weather and the season. They come out early in the morning, and after rain. Some bury themselves in moist places during the dry season, or burrow under leaves, gra.s.s, or stones, often closing the mouths of their sh.e.l.ls with a white secretion to prevent evaporation during the period of hibernation.

The smallest sh.e.l.ls, especially of land species, and young imperfect sh.e.l.ls should be collected.

In all cases "live sh.e.l.ls," _i.e._ sh.e.l.ls in which the animal is alive, are to be chosen; but, when these cannot be procured, "dead sh.e.l.ls," which have not lost their l.u.s.tre, or their colour, especially those of rare species, should be preserved.

_With regard to the mode of Preserving Sh.e.l.ls._

1. No attempt should be made to clean them, or to remove the furry skin, more or less thick, with which they are often covered, beyond removing with a soft brush any mud or sand adhering to them.

2. The animals of Land and Freshwater sh.e.l.ls may be killed by immersing them for a few minutes in _boiling_ water, after which the bodies may be easily extracted whole with any suitable instrument, _e.g._, a fork or a pin, according to size. Hot water should not be used with marine sh.e.l.ls: it often destroys their l.u.s.tre. They should be buried, if time permits, in sand, or other dry material, until the animal dries up (in small sh.e.l.ls) or rots (in large specimens); or they may be drowned in cold fresh water, and hung up in the air to dry or rot away. In the former case, if an operculum (with which some species, both marine and land, close their mouths, more or less partially) exists, it will, generally in the case of land sh.e.l.ls, remain in its place, adhering to the sh.e.l.l. In the latter, the decayed matter should be washed out, and the operculum, if any, replaced and fixed, say, on cotton filling the sh.e.l.l. This applies equally to land sh.e.l.ls.

3. Care should be taken not to injure the edge or lip of the mouth of univalves, or the ligament of the hinge of bivalves. When bivalves gape on dying in water, or if the ligament be broken, the valves should be closed and tied together. If the ligament of a gaping bivalve should become dry and stiff, it can be softened by putting it in water.