Part 8

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, Sphærium and Pisidium.

Sphærium corneum.--Very common. Shell 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.

Sphærium 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.

Sphærium pallidum.--Local in canals and ponds. Oblong. Distinguished also from the previous species by the body being milk-white, and the shell is ashy-grey.

Sphærium lacustre.--Local. On the beaks is a calcareous nucleus which distinguishes it. It is thinner than corneum, and rounder than pallidum.

Pisidium amnicum.--(Our five pisidia resemble Sphærium, but are much smaller, all but amnicus being minute. Very abundant where found. P. amnicum 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. amnicum 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. amnicum.

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 shell 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 Mytilidæ (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. Shell 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.

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All our shells 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 shells 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 shells 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 shells in a collection before a regular cabinet is obtained, the tinier shells may be kept in small glass tubes with corks (such as used for homoeopathic medicines), and the medium sized ones in the trays of common matchboxes, these being arranged in large shallow glass-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 substitute for a more costly cabinet. In all cases the name, and the place where the shells 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 shells 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 SHELLS OF MOLLUSCS.

The following notes supply a few general rules as to finding and preserving shells:--

Of Shell-bearing Molluscs there are three classes--Marine, Freshwater, and Land. The first two include Univalves and Bivalves, the last only Univalves.

1. MARINE SHELLS 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 shells can be obtained; but after a storm these may often be found upon the shore, before they have lost their lustre.

Limpets, etc., should be detached with a thin blade passed quickly under the shell, taking care not to break the edges. Small shells 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 SHELLS 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 SHELLS.--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, grass, or stones, often closing the mouths of their shells with a white secretion to prevent evaporation during the period of hibernation.

The smallest shells, especially of land species, and young imperfect shells should be collected.

In all cases "live shells," i.e. shells in which the animal is alive, are to be chosen; but, when these cannot be procured, "dead shells," which have not lost their lustre, or their colour, especially those of rare species, should be preserved.

With regard to the mode of Preserving Shells.

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 shells 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 shells: it often destroys their lustre. They should be buried, if time permits, in sand, or other dry material, until the animal dries up (in small shells) 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 shells, remain in its place, adhering to the shell. In the latter, the decayed matter should be washed out, and the operculum, if any, replaced and fixed, say, on cotton filling the shell. This applies equally to land shells.