Any of various sand dollars having one or more elongated perforations in the test and belonging to the family Mellitidae or related families; especially one of the genus Mellita of the Atlantic coasts of America.
Origin
Mid 19th century.
Definition of keyhole urchin in US English:
keyhole urchin
noun
Any of various sand dollars having one or more elongated perforations in the test and belonging to the family Mellitidae or related families; especially one of the genus Mellita of the Atlantic coasts of America.