part formed by the closure of protoconch II after loss of protoconch I, in the centre of the caudal shell margin in Diacavolinia. The species in this genus first develop a bowl-shaped embryonic shell (protoconch I) on which a conical part is formed; very quickly this part broadens and a transverse slit starts to separate the dorsal and ventral side. All soft parts retract into this "bivalve-like" shell and the embryonic shell with the conical part is shed; the opening left at the underside of the "bivalve-like" part is closed by bending of the ventral and dorsal side and secretion of extra shell material, but no separate septum is formed; this fusion gives rise to the joint.
Compare: caudal joint; caudal fold; caudal septum; caudal spine.

Alternative form for caudal joint : closing fold.