monocyclic crinoid cup
This is a diagram of a monocyclic cup
unwrapped and flattened out. The arm bearing plates
have notches called "arm facets" from where the arms
extend. These plates are known as "radials", the
smaller plates above the radials are the "orals", and the
lower plates are "basals". The oral plate with the
circlet of triangular plates is the
anal pyramid. The two shaded
plates are part of a series of anal plates supporting
the anal pyramid, and are known as the "anal X"
(the lightly shaded plate), and the "radianal"
(the darker plate).