Terminology of key morphological characters for Theodoxus. A. Shell: a = apex, ap = aperture (im = inner margin, om = outer margin), cp = columellar plate, p = periostracum, s = spire, sh = shell height, sm = shell margin, sw = shell width, w = whorls; B. Operculum: ap = apophysis, ca = callus, cb = calcareous base, cl = conchioline lamella, la = left adductor, pa = pseudo-apophysis, ra = right adductor, rp = rib-pouch, rs = rib-shield; C. Radula: ac = A-central (c = cusp, r = ridge, tp = tooth plate), bc = B-central, cc = C-central, el = E-lateral (le = lower edge, ue = upper edge), mt1 = first row of marginal teeth (sf = small face of marginal teeth), mt2 = second row of marginal teeth, rc = R-central (ae = anterior edge, f = tooth face).