Something in the stream that looked vaguely mussel-like had been catching my eye for a while now. It was near our foot bridge, so I would look down at it every day and wonder what kind of trash it was that was half buried in the streambed. Today I noticed that the crack/gap down the middle was closed for the first time, so I was finally inspired to investigate. I was surprised to discover that it is a real, live mussel!
According to the King County website, we have a freshwater mussel in our area called the Western Pearlshell (Margaritifera falcata). I believe that is what I've found. Here is a link to the King County Freshwater Mussels homepage. According to what I read there, these mussels are a good indicator of stream health because of their sensitivity to pollutants. So finding one is good news, though I'm sure finding a bed full would be even better...
The Western Pearlshell pictured on the King County website appears to be about the size of the one that I found, and the information says that the one in their picture is estimated to be around 50 years old!
I'm a little worried about the look of the hinge area of this one -