rudist
Concise Oxford English Dictionary © 2008 Oxford University Press:
rudist /ˈruːdɪst/
▶noun a cone-shaped fossil bivalve mollusc which formed colonies resembling reefs in the Cretaceous period.
– origin C19: from mod. L. Rudista (former group name), from L. rudis ‘unwrought’.
'rudist' also found in these Oxford entries:

