What is (Sabellidae)? Comparisons of populations from Britain, Iceland and Canada with comments on and .

A neotype of Amphitrite Müller (1771) is established and re-described, using material collected in 2001 from near the type locality on the southwest coast of Iceland. We examined material from Iceland, Nova Scotia and Newfoundland, identified as Pseudopotamilla reniformis, showing scissiparity and commonly found in aggregations within kelp holdfasts or amongst encrusting fauna. By contrast, scissiparity has not been seen in a population recorded as P.