Native littleneck clam on the left, Manila steamer on the right. Manilas were introduced to Puget Sound along with the Pacific Oyster in the 1950s and, like the oyster, have since naturalized. They're now the main clam species grown and harvested commercially in the Puget Sound, as their shells are hardier and they last longer post-harvest than the native littleneck. How to tell them apart? In general, the native clams are lighter colored than Manilas, but the biggest difference is in the profile of the shell. As you can see from the photo above, the native clams are more circular in shape.
Two Manilas, one littleneck.
Cockle clams are extremely beautiful, and occasionally eaten, but aren't harvested commercially. 
Pretty cockle shells, all in a row. These cockles are called Nuttall's cockles, or, more whimsically, heart cockles, so-called because when you look at the clam from the side the shell resembles a (somewhat squashed) heart. Go here for a picture of a live heart cockle. Heart cockles don't live as deep in the sand as most other clams do. Instead they rely on their thick, ribbed shell to protect them from predators. They also have sensory organs along their mantle (visible in the link above) that can detect approaching predators, and they use their foot to drag themselves to safety. If they're lucky, they can live up to 15 years.
Oysterfan:
We rake up both the Manila and the Littleneck on our beach. We also have what we think is the Mahogany clam.
http://faculty.washington.edu/cemills/Nuttallia.html
We avoid those because we can’t seem to get the sand out of them with the traditional corn meal approach. I chipped a front tooth earlier this year on grit from one of those little purple rascals!
Sue would like to know if you have any recipes for the cockles on Hood Canal? Any recipe we find online seems more suited to small cockles and not the big ones found here.
Hope you are well…..we are heading to Frenchman’s Cove Friday afternoon.
Bruce
Hi Bruce,
We also have the Mahogany clam… although we call them varnish clams. I’ve got a varnish clam post in the hopper. And I’ve never eaten cockles! I’ve been meaning to… I’ll let you know how it goes.