Polycera quadrilineata (O. F. Müller, 1776)
1: no elongate processes at base of rhinophore.
2: single, not strongly-curved, backward-pointing, linear process at each side of the gills.
3: anterior of foot expanded into unobtrusive, small, propodial tentacles.
4: four or more linear processes on the anterior edge of the mantle over the head.
Length 20 mm. North Wales. February 2011.