On the morphology and life-history of Steringotrema pagelli (van Beneden, 1871) Odhner, 1911 and Fellodistomum fellis (Olsson, 1868) Nicoll, 1909 [Syn.S. ovacutum (Lebour, 1908) Yamaguti, 1953] (Trematoda, Fellodistomidae) - Køie, M. (Ophelia, 1980)