Leucophytia bidentata
Left: three views of 4.1mm high shell occupied by contracted, cream, live animal seen through translucent white shell.
Right: two vacant shells, heights 4.3mm and 3.8mm.
Usual adult height to 5 mm occasionally to 7 mm.
Inner (columellar/parietal) lip has only 2 protrusions within the aperture.
Outer (palatal) lip has no protrusions or rib (sometimes in a photo, a strong growth line might be mistaken for a rib).
Menai Strait, Wales. April 2018.