Ixa cylindrus, which has no common English name I could find easily. According to this site, they are called “Kabuto-kobushi” in Japanese.
My suspicion is that these wide extensions of the carapace help to prevent it from being eaten by predators by making the crab wider than a predator’s mouth. My only shortcoming in that hypothesis is, if that were so, why wouldn’t they be sharp?
I'll hazard a guess: they aren't sharp because if you get stuck in a predator's mouth you are just as dead as if it had eaten you. You want to block being swallowed but still be spit-out-able.
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