A slender fish, deepest about two fifths of the way along. The upper jaw projects further than the lower and the snout overhangs the mouth. There is a small barbel on the fleshy lower lip. The dorsal fin is divided into two parts. The number of spines and soft rays in the fins is indicative of the species and in M. americanus, the front part of the dorsal fin is broadly triangular and has 10 spines and the other part is long and has 1 spine and 22 to 25 soft rays.