Shāqián shíjìshang shì yī zhǒng biǎnpíng de hǎidǎn.
Sand dollars are actually a type of flat sea urchin.
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Calque of English 'sand dollar' - the dried white skeleton is round and flat like an old silver coin, with a five-petal pattern on top. (In Classical Chinese 沙钱 can also mean a poor-quality copper coin - context disambiguates.)
