The lunula (also known as a persisting center of the ulnar styloid and (I think) os ulnostyloideum) is an ossification center within the fibrocartilaginous structure between the triangular fibrocartilage, triquetrum, and the base of the fifth metacarpal. It is found distal to the ulnar styloid between the triangular fibrocartilage and the triquetrum.
It may fuse to ulnar styloid and give it an unusual and long appearance. It may also be difficult to distinguish from an old ulnar styloid avulsion (which may be the case in the image shown).
The lunula is a common finding in non-human primates.
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