Modiolus capax, common name fat horsemussel, is a species of "horse mussel", a marine bivalve mollusc in the family Mytilidae, the mussels. It was first described to science by American malacologist Timothy Abbott Conrad in 1837. The type specimen was collected in San Diego by Thomas Nuttall.

Fossils of Modiolus capax have been found in Miocene epoch deposits, suggesting that this species is between 5.3 and 20.4 million years old.

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