Perhaps this is a calyx. Good eye to pick up this subtle detail. However it is too indistinct to be sure it is a calyx. Check out some images of better preserved calyx at:https://www.google.com/search?q=crinoid+calyx&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi43eLPlP7YAhXHg-AKHX30CVoQ_AUICigB&biw=1280&bih=630#imgrc=_
Jim