Atherion elymus has an elongated, compressed body with a small mouth in which the upper jaw does not extend as far as the front edge of the eye. The head has a number of rows of denticles or small spines. Its anus sits immediately in front of the origin of the anal fin. The colouration is greenish gray on the back and whitish on the underside with a wide silvery band along the flanks which extends to the caudal fin. Small dark or black spots are normally scattered along the lower part of the flanks. They grow to a maximum standard length of around.
Atherion elymus is a species if shallow water and is associated with reefs, large aggregations can be found in tidal pools, along rocky coastlines and along the edges of reefs. It lays large eggs which adhere to the substrate by a sticky filament.