In the present paper a study has been made of the genus Ensis in Europe. Seven species and three varieties are recognized (see p. 18), the following of them being described as new: E. minor (Chenu) var. subarcuata (p. 27), E. arcuatus (Jeffr.) var. ensoides (p. 31) and var. norvegica (p. 33), E. phaxoides (p. 39) and E. sicula (p. 42). Though E. magnus Schum. has been separated here from E. arcuatus (Jeffr.) as specifically distinct, it may actually form a dine with the latter. As to the related Atlantic North American species of the genus it is pointed out, that the species commonly called E. directus Conrad should probably be named E. americanus Binney, while E. minor Dall (1899) seems in need of a new name, since it is unfortunately antedated by E. minor (CHENU, 1843).