A shadow over the cup of an upturned cockle shell. Shadows also set off the fluting along the margin. This is the shell of the heart cockle, Clinocardium nuttallii.
I like the shifting influence of sunlight and shadows on the ribs radiating over the cockle’s curved external surface. Next time you come across a complete heart cockle shell, see if you count 35 ribs.
If this post hasn’t satisfied your cockle curiosity I’ve just opened a Bivalves page where you can see more images of heart cockles (and even lean why they’re called heart cockles) and get my take on a few other common bivalves.