In the typical genus (Tellina), of which we represent two very common British species, the ligament is very prominent, and the slender siphons are often much longer than the shell. The members of this group move very freely, travelling about by means of a broad, flattened foot.
Fig. 141.—Tellinidæ
1. Psammobia ferroensis. 2. Donax anatinus. 3. Tellina crassa. 4. Tellina tenuis. 5. Donax politus
The shells of the genus Psammobia are popularly known as Sunset shells, being prettily marked with radiating bands of pink or other tint, reminding one of the beams of the sun when setting in a cloudy sky. In these, too, the ligament is very prominent, and the shell gapes slightly at both ends.
The same family contains the pretty little Wedge shells, which are so called on account of their triangular form, and constitute the genus Donax. These shells, which are seldom much over an inch long, are very common on some of our sandy beaches, being washed up in considerable numbers after the animals have died, but the specimens are seldom perfect. The molluscs themselves are burrowers, and live in the sand, at and just below low-water mark; and, as they usually burrow to a depth of only a few inches, are easily obtained alive.
The shells are rather thin, closed at both ends, blunt and rounded at the anterior end, but straight and more pointed at the shorter posterior end; and the margins of the valves are very finely grooved in such a manner as to resemble the milling of a coin. Each valve has two central hinge teeth, with one long lateral tooth on each side; and the ligament is external and prominent. The lobes of the mantle are fringed; the siphons are separate and diverging, but shorter and thicker than in most of the other Tellinidæ, and the foot is comparatively large, flattened, and pointed.
The genus contains many species, the commonest being, perhaps, D. anatinus, the colour of which is yellowish, banded with brown, and marked by a number of radiating white lines. This colour, however, is due entirely to the thin, shining epidermis that completely covers the valves; and if this is rubbed off the shell itself will exhibit a pale pinkish tint. Another common species (D. politus) may be recognised by the broad patch of white running from the hinge to the margin, on the posterior side of the middle of each valve.
The family Mactridæ contains some British shells popularly known as Trough shells, and the family name itself is derived from the word mactra, which signifies a kneading trough. In this group the shells are all more or less triangular in form, with the valves equal, and are either closed or very slightly gaping. The ligament, perhaps more correctly designated the cartilage, is generally internal, and contained in a deep triangular hollow; and the shell is covered with epidermis. The mantle of the animal is open in front, and the siphonal tubes are united and fringed. The foot is usually large and flattened.