It was only ~1 cm long, and when we looked at it under the microscope we could see a number of interesting features.
Here's a view of the head end (below). Note that the girdle — the fleshy portion encircling the eight valves or plates — is smooth and the valves themselves appear pitted or granular.
At the tail end there were a few isolated setae, or hairs (see next photo, with the chiton out of the water for better visibility of the setae). It was notable that the setae were confined to the posterior end, as many other chitons have setae that extend along the entire perimeter of the animal.
When identifying chitons, it's important to check whether the setae have bristles or not, and if so, to look at the shape of the bristles. The next image shows a close-up of one of the setae:
Ah ha! The setae did have bristles...there was just one row of them...and the bristles were recurved (arching back towards the chiton). They looked like a pretty good match for this:
Modified from Marine Invertebrates of the Pacific Northwest by Eugene Kozloff (1996)
All of these characters led us to believe that this chiton was Lepidochitona flectens (formerly Dendrochiton flectens and Basiliochiton heathii).
Most books describe Lepidochitona flectens as relatively rare, occurring primarily in subtidal areas (5-10 meters deep), but also in the low intertidal zone on rocky shores. Their range extends from British Columbia to Monterey Bay, so it's a more northern species. They're usually less than 2 cm long, so I'm guessing Lepidochitona flectens is easily overlooked — which is too bad because its flecks of color are something to behold!