Height in Croatia usually 4mm to 6 mm (J.Prkić, in litt.), maximum c. 8 mm . Often a slight tooth-like protuberance on the columella (1). Entire lip everts to form a flared aperture on mature adults.
Well developed ribs overhang suture (2) to give a tiered flounced effect that is unusually strongly developed on these Croatian specimens, and somewhat resembles R. paradoxa which occurs on the north African coast.
LEFT: Mediterranean form. Shell heights 3.67mm and 5mm. Beached specimens. Split, Croatia. Leg. © J. Prkić. RIGHT: Atlantic form. Shell height 5.6mm. Beached. Silverstrand, Galway, Ireland. September 2002. Leg. J. Fisher
See www.fotoconchigliemediterraneo.com/2013/02/in-allestiment… for a wide selection of Mediterranean rissoids, including R. paradoxa. Some of the names are not recognised on WoRMS and the site requests corrections for errors.
Wigham (1975) wrote:
“The classification of the Rissoidae has been, and continues to be, based largely upon shell characters. In a group of small animals of variable form this has resulted in a state of considerable taxonomic confusion”