34 America}! Seashells 



sists of a very thin layer of conchiolin, probably not more than 0.003 n^"^- 

 in thickness. In other clams, such as Area, 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: (i) The outer layer of shell which 

 consists of elongate radial prisms of calcite. These are arranged in concen- 

 tric 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 Pi72na 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 in- 

 crease takes place during the first year or two. A species may show con- 

 siderable variation in its manner of growth under different living conditions 

 at various localities. Thus the Pacific Razor Clam (Siliqiia 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 {Clinocardhmi nuttalli) which in ten years 

 grows to three inches in length in California, but in Alaska it survives six- 

 teen 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 calcula- 

 tions. The average age of the Atlantic Bay Scallop {Aeqiiipecten irradians) 

 is about sixteen months, its maximum age only two or three years. The 

 average age of a five-inch Pismo Clam (Tivela stidtonim) on the Pacific 

 Coast is about eight years, its maximum age twenty-five years. The Com- 

 mon Blue Adussel (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 {My a 

 arenaria) takes about five years to reach an edible size of three or four inches 

 and may live for ten years. The AVashington Clam {Saxidounis imttalli) lives 

 ten to fifteen years or longer, while Nuttall's Gaper Clam {Schizothaenis 

 nuttalli) may survive for seventeen years. 



