There are ~2000 different kinds of clams but only two main groups sold commercially, softshelled and hard-shelled. Soft-shell does not mean the clam has a shell that is soft to the touch, rather it refers to the clams with thinner more brittle shells. Hard-shelled clams have a strong shells and can tolerate higher salinity. Hard-shelled clams are found in tidal areas along the east coast of the US and Canada and the west coast of the UK. There are also hard-shelled clams in the US Pacific Northwest called manilas. They are an invasive species. There are two varieties of US East Coast hard-shelled clams referred to as quahogs (the Indian word for clam), but the clams are more commonly named according to their size/age. The size of a clam is a measure of the width across its hinge or the thickness. As a clam gets older, it grows larger. It is important to note that vendors may have different grading systems. The smaller sizes are usually farmed-raised, while the larger sizes tend to be wild product.