Clams...
Clams, such as the northern quahog, are bivalves, meaning that they have shells consisting of two halves, or valves. The valves are joined at the top, and the adductor muscles on each side hold the shell closed. If the adductor muscles are relaxed, the shell is pulled open by ligaments located on each side of the umbo.

The clam's foot is used to dig down into the sand, and a pair of long siphons that extrude from the clams's mantle out the side of the shell reach up to the water above (only the exit points for the siphons are shown). Clams, such as the northern quahog, are filter feeders. Water and food particles are drawn in through one siphon to the gills where tiny, hair-like cilia move the water, and the food is caught in mucus on the gills. From there, the food-mucus mixture is transported along a groove to the palps which push it into the clam's mouth. The second siphon carries away the water. The gills also draw oxygen from the water flow.

The mantle, a thin membrane surrounding the body of the clam, secretes the shell. The oldest part of the clam shell is the umbo, and it is from the hinge area that the clam extends as it grows.

Types of Clams
There are many types of clams offered for consumers and the names and locations can be confusing.  So this information might help you understand what you are looking at in the marketplace.  There are two basic types of clams: hard-shell clams and soft shell clams.

Hard-shell Clams
In California you might be offered many sizes and types of hard-shell clams. You basically have to choose from an east coast clam, a west coast clam or (in our summer time) a New Zealand cockle clam. 

The east coast produces two varieties of hard shell clams, the northern quahog or the southern quahog clam. (Quahog is an Indian word for clam.  By the way, clam shells were used as MONEY by the Indians) the northern clam (Mercenaria mercenaria) is most commonly harvested around the Great South Bay (New York), Chesapeake Bay and north to Canada.  The southern clam (Mercenaria campechiensis) is a product mostly of aquaculture and is grown fro North Carolina to Florida, the Indian River area of Florida being a major producing area.  Northern clams have a dull, light brown color and southern clams have an “Indian blanket” pattern on their shells.  There are some farms producing ‘triploid’ clams, clams that cannot spawn.  This allows the grower to sell clams year round.  (That is what that non-R month stuff is about – sewed up shellfish).

The west coast produces two species of hard shell Pacific clams (sometimes called Manila clams).  These two species of clams are mixed up in the west coast clam beds.  Almost all the production is from Washington state and Canada.  These clams have a shorter shelf life then eastern hard shell clams (their valves do not close as tight) but come to the market faster then eastern hard shell clams.  These clams have thinner shells, which means you get more clams in a pound of clams as compared to the eastern clams.

The New Zealand cockle clam is a Pacific clam native to the Southern Hemisphere.  It is flown in when our summer causes Northern Hemisphere clams to spawn, making them weak and having little to no meat inside.  (Spawn till you die!) Their greenish colored shells easily recognize them.

Soft shell clams
Unless you are from the northern east coast or well traveled, most people are ignorant about soft shell clams.  There are different types of soft shell clams but the two types most commonly encountered are the east cost soft shell clam (New England steamers, Ipswich clams or piss clams) and the west coast Goeduck clam (used in chowders, clam burgers and sushi bars).  All soft shell clams have an external skin on their necks that must be removed before eating and this freaks out most consumers (it looks like a penis). Eastern soft shell clams are often shucked and used in fried clam dishes. (Howard Johnson’s was busted for its fried clam menu offering since the clams were arranged in a subliminal orgy. The dish was wildly popular). Soft shell clams have an extremely short shelf life because they do not close tightly like hard shell clams. The Goeduck clam is farmed and wild harvested on the west coast, the neck meat is used in sushi bars and the body meat is used in chowders and clam burgers.  Both these clams are very ‘sandy’ inside and should be purged before using.  Most producers purge the clams before shipping.  Goeduck clams are rather phallic, with necks that measure from 12 to 30 inches long.  Think super-soaker 50 and you get the picture.  Goeducks can sell for as much as $25/clam and weigh as much as 5 pounds.  The shells are light.  Digging goeducks for sport harvesters is an adventure and if you get two in one day, you are lucky and tired.

Thanks to The Assateague Naturalist website for some of this information.