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34 of a very thin layer of conchiolin, probably not more than 0.003 mm. in thickness. In other clams, such as Arca, it may be many times as thick. It is normally secreted from a group of cells situated just under the tip of the mantle. The three shell layers are: (1) The outer layer of shell which consists of elongate radial prisms of calcite. These are arranged in concentric bands which are plainly visible on the outer surface of the shell. (2) The middle layer which is entirely composed of aragonite in the form of “crossed lamella.” This specialized structure is peculiar to mollusks. (3) The inner layer which is a homogeneous layer of porcellaneous material. The Pen Shells of the genus Pinna commonly found on the west coast beaches of Florida offer an excellent demonstration of prismatic structure. When the surface is examined with a high-powered lens, it appears to be honeycombed. What you see are the ends of the needle-like prisms of calcite which, although closely packed together, are separated from each other by a thin varnish of conchiolin. By examining the edge of the broken shell you can make out the prisms in side view.

Most clams continue to grow in size during lifetime, but the greatest increase takes place during the first year or two. A species may show considerable variation in its manner of growth under different living conditions at various localities. Thus the Pacific Razor Clam (Siliqua patula) in its southern range in California grows much faster and reaches a length of about five inches in three years. In Alaska, however, it grows more slowly, taking five to eight years to reach the same size. Yet the northern colonies continue to grow for a greater length of time, some living for fifteen to eighteen years and eventually reaching a length of over six inches. This is also true of the Pacific Cockle (Clinocardium nuttalli) which in ten years grows to three inches in length in California, but in Alaska it survives sixteen years to reach a length of five inches.

The maximum age is known for a few species of clams. It is believed that the giant Tridacna clam of the Indo-Pacific lives for perhaps a hundred years, but this has not been confirmed by experiments or accurate calculations. The average age of the Atlantic Bay Scallop (Aequipecten irradians) is about sixteen months, its maximum age only two or three years. The average age of a five-inch Pismo Clam (Tivela stultorum) on the Pacific Coast is about eight years, its maximum age twenty-five years. The Common Blue Mussel (Mytilus edulis) grows to about two inches the first year, to four inches the second year but, beyond this, it grows very little although it may live for a total of seven or eight years. The Soft Shell Clam (Mya arenaria) takes about five years to reach an edible size of three or four inches and may live for ten years. The Washington Clam (Saxidomus nuttalli) lives ten to fifteen years or longer, while Nuttall’s Gaper Clam (Schizothaerus nuttalli) may survive for seventeen years.