In this paper an account is given of new forms discovered by the writer during a survey of the parasites of mussels and cockles, obtained chiefly from the estuary at Conway, but also from other beds on the North Wales coast. The trematode parasites of shellfish in this area have not previously been dealt with, although several forms have been described from the Liverpool Bay area by the late Prof. Johnstone and his colleagues. Few additions have been made to our knowledge of the early stages of British marine Trematodes since the publication of a series of papers by Lebour (1912) and Nicoll (1914).