That is the skeleton of a burrowing sea urchin in the genus Brissus. Probably Brissus latecarinatus. Sometimes called a sea mouse, sometimes a heart urchin. In life they are covered with short, thin, not particularly sharp spines. You can usually handle them safely. Live ones stay buried in the sand nearly all the time, unless chased by a carnivorous helmet shell, which sucks out their juices. There are quite a few different kinds but with somewhat different shapes. They are related to the sand dollars, which are much flatter.
