In the NarthecoceratidaeFlower (1958) there is variation of structures within the tubes of the endosiphuncle which may be empty, or may contain endocones or “bracket diaphragms”,Winstonocera teicherti gen. et. sp. nov. shows numerous diaphragms from which tubes extend a short distance forward. It is suggested that the “bracket diaphragms” are similar structures. It is further suggested that the structures within the tube were originally formed in soft tissue, which was later calcified. The same appears to apply to endoceroid endosiphuncles.Narthecoceras glenisteri is the firstNarthecoceras to be described from the Stewartville Limestone of the northern Mississippi Valley.