Did you guess part of a crab? If so, you're right! Here's the entire object.
This is the carapace (basically the "back") of a Spiny Mole Crab (Blepharipoda occidentalis). I found it washed up at Doran Beach on 1 November 2013. It was ~2 inches (5 cm) long.
Most people are familiar with a more common species of Mole Crab (Emerita analoga). They're smaller, grayer, more rounded or domed, and lack the prominent spines. Here are the two side-by-side for comparison (see next image).
When you find just the carapace alone and not the entire crab, it's often a molt, which means that the animal has shed its old exoskeleton (that's what you're looking at) but has grown a new exoskeleton and is still alive.
We moved to Sonoma County in December 2004 and were excited to find Spiny Mole Crab molts at Doran Beach and Salmon Creek Beach. When you read about the distribution of Spiny Mole Crabs in books, you may see their northern range limit described as San Francisco, Stinson Beach, or Drakes Beach. So it's quite possible that the observations at Doran Beach and Salmon Creek Beach are the northernmost records for this species in California.
In 2005, I found a live Spiny Mole Crab at Doran Beach. They live in the very low intertidal zone and subtidal zone, so it's harder to find them than the more familiar Emerita analoga.
For site documentation purposes, finding a live crab is better than finding a molt because a molt could wash in from farther away. To say that Spiny Mole Crabs regularly occur at Salmon Creek Beach, it would be good either to find live Spiny Mole Crabs there, or to locate additional molts.
This past week I had a strange experience in that I was remembering the Spiny Mole Crab molt that Eric and I found on Salmon Creek Beach in 2004. I haven't seen one there since. I haven't done a purposeful search for them, but I keep my eyes open as I'm walking the beach.
Well, after I found the molt at Doran Beach on November 1st, I went for a quick walk on Salmon Creek Beach on November 2nd, and what do you know, there was a Spiny Mole Crab molt on Salmon Creek Beach (see picture below)!
We still don't know if there are Spiny Mole Crabs actually living at Salmon Creek Beach, so it'll be interesting to see if we can find a live crab there some day. (Or let me know if you or anyone else you know has found one there or even farther north!)