According to my chemistry book (Chemistry: The Molecular Nature of Matter and Change by Silberberg) seashells including clam shells are primarily composed of Calcium Carbonate.
A clam shell
Possessive form of the words clam shell: clam shell's
The mantle of a clam protects the clam from sunlight in shelled mollusks, the mantle is the organ that forms the shell, and adds to the shell to increase its size and strength as the animal grows.
A clam. Or an oyster.
The largest shell belongs to the giant clam, Tridacna gigas, whose shell can grow to several hundred pounds, with a length of about 5 feet.
A clam shell grows via a process where tissue is attached to the shell at the edge. Over time this material builds up producing a ring like pattern. The number of rings will indicate the age of the clam shell.
The mantle of the clam lines the valves, and serves to secrete calcium carbonate in the formation of the clam's shell while protecting the clam's visceral mass.
By it's shell
Only if you poo on it
The umbo which is located at the anterior end of the clam
A clam shell is not smooth because being smooth would decrease friction between a clam and the surface it lives on. With low friction, a clam would go places it would not want to go.