Anomia ephippium is an irregularly shaped bivalve with a thin shell, found attached to stones and to other bivalve shells. It is approximatelty circular and up to 60 mm in diameter. The right valve is lowemost and adjacent to the substratum, the left valve is uppermost. It attaches to the substratum by byssal threads though a hole in the right valve. The right valve often takes on an impression of the substratum and this is most obvious when an individual has been attached to a scallop. The edge of the left valve is often shaped to fit exactly onto the substratum. The shell is very thin and brittle, usually white with a blue or pink tinge. The left valve has irregular growth lines on its surface although these can be worn off in older individuals.The four species of saddle oysters are easy to recognise as a group but it can be difficult to separate the species. Anomia ephippium is distinguished from other saddle oysters (Anomiidae) by a line of three muscle scars on the inside surface of the convex valve.
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