1911 Encyclopædia Britannica/Miller's Thumb

MILLER’S THUMB (Cottus gobio), a small fish, abundant in all rivers and lakes of northern and central Europe with clear water and gravelly bottom. The genus Cottus, to which the miller’s thumb belongs, is easily recognized by its broad, flat head, rounded and scaleless body, large pectoral and narrow ventral fins, with two dorsal fins, the anterior shorter than the posterior; the praeoperculum is armed with a simple or branched spine. The species of the genus Cottus are rather numerous, and are confined to the north temperate zone of the globe, the majority being marine, and known by the name of “bullheads.” The miller’s thumb is confined to fresh water; and only one other freshwater species is found in Europe, C. poecilopus, from rivers of Hungary, Galicia, and the Pyrenees; some others occur in the fresh waters of northern Asia and North America. The miller’s thumb is common in all suitable localities in Great Britain, but is extremely rare in Ireland; in the Alps it reaches to an altitude of 7000 ft. Its usual length is from 3 to 5 in. Generally hidden under a stone or in a hollow of the bank, it watches for its prey, which consists of small aquatic animals, and darts when disturbed with extraordinary rapidity to some other place of refuge. The female deposits her ova in a cavity under a stone, whilst the male watches and defends them until the young are hatched and able to shift for themselves.