I was thinking this was a fissurelid, but check out the shell. It has a little peak, almost a hook like Crepidula. Pretty chunky, maybe 4-5 cm in this pic. Turns out its something totally different: http://www.seaslugforum.net/find/umbrumbr. ID'd with nudipixel: http://www.nudipixel.net/species/umbraculum_umbraculum/2/