[typical culinary species of the four most important genera:
Atlantic Jackknife Clam; Ensis directus (North America, now invading Europe)
Pacific Razor Clam; Siliqua patula (Alaska to Pismo Beach, California)
Razon Shell; Ensis arcuatus (Eastern Canada, Northern Europe)
Gould's Razor Clam; Solon strictus (East Asia)]
All the genera listed above look much like the photo specimens, except the Pacific (Siliqua patula) which is much shorter. Most are Northern Hemisphere clams, but one species, Navaja (Ensis macha), inhabits both coasts of South America and is a significant commercial catch in Chile. They feature a digging foot at one end and a double siphon at the other, Razor clams are very highly regarded as food but are difficult to catch because they can dig down faster than a person can dig them up, and some can dig down more than 40 feet.
The photo specimens were purchased live at a large Asian market in Los Angeles at 2013 US $5.49/pound. They may be Ensis directus or an almost identical species. The longest was 5.88 inches long and 7/8 inch wide. They typically weighed 1.63 ounces and yielded 0.63 ounce steamed edible (39%), or $14.08 per edible pound. That's not at all bad for shellfish around here. The open photo specimen at right in the photo has been steamed.
More on Bivalve Molusks.
Buying: These are not easy to find here in Southern California, but as noted above, Asian fish markets may have some now and then. I've seen the live ones for the first time in September 2013, but certainly hope they become a regular item. They are a much better value than the frozen ones, both for cost per pound and flavor.
Cooking: Steam them lightly and eat them plain or with a very light dip. You don't want to screw around with something so simple and so delicious. Of course you can also use them in seafood soups. Steam lightly first (to make them easy to shell), shell and cut. Add to the soup in the last minute to avoid overcooking.
The live specimens (photo at top) were tender and delicious with a distinctly briny flavor. They are very meaty due to the powerful digging foot which occupies most of the shell. Note the one at the right that has been steamed. The dark splotch at left center is the stomach, which is easy to clean out if you want to bother. Clams are usually eaten stomach contents and all.