a. A European fish (Thymallus vulgaris), allied to the trout, but having a very broad dorsal fin; -- called also umber. It inhabits cold mountain streams, and is valued as a game fish. a. An American fish of the genus Thymallus, having similar habits to the above; one species (T. Ontariensis), inhabits several streams in Michigan; another (T. montanus), is found in the Yellowstone region.