The typical genus, Mactra, contains some common molluscs that bury themselves just beneath the surface of sandy beaches; and these are so abundant in some parts of Great Britain that they are used largely for feeding pigs. Some of the mactras are remarkable for the great power and extensibility of the foot, which, in some cases, is used so vigorously that the animal turns itself quickly over, or even leaps on the ground.
Our example of this genus is M. stultorum, which is a very common object of the shore. Its colour is very variable, usually some shade of grey or brown, and marked by radiating white lines.
The Otter shells (Lutraria), of which we figure one species, are much like the Mactræ in structure, and are usually included in the same family, but in some respects they resemble the Myacidæ or Gapers. The shell is oblong rather than triangular, and gapes at both ends; and the animal buries itself deep in sand or mud, principally in the estuaries of rivers, from low-water mark to a depth of about ten fathoms. The shells are not very common objects of the shore, for they are found only in muddy places, and those of the commonest species (L. elliptica) are too large and heavy to be washed ashore in the sheltered estuaries where they abound.
Fig. 142.—1. Lutraria elliptica. 2. Part of the Hinge of Lutraria, showing the Cartilage Pit. 3. Macra stultorum. 4. Interior of same showing Pallial Line
We now leave the burrowers, to consider a family of molluscs that move about somewhat freely by means of a flattened tongue-shaped foot, and which only rarely fix themselves in any way. The shells of the group are popularly known as Venus shells, probably on account of the beauty of some of the species, and the family in question as the Veneridæ.
The shells of the various species are usually of a graceful oval or oblong form, frequently marked by chevron-shaped lines in pretty colours, and distinctly grooved along the lines of growth. The ligament is external, the hinge has usually three diverging teeth in each valve, and the pallial line is sinuated.
The principal genus is Venus, in which the shells are ovate in form, thick, and smooth, and the margins of the valves are minutely crenulated. The genus is a very large one, and contains several British species, two of which we represent in the accompanying illustrations.
Allied to these is the larger but pretty shell Cytherea chione, which inhabits deep water off the southern coasts, to about one hundred and fifty fathoms. It is much like the Venus shells in form, but the margins are not crenulated.