Acestridium was diagnosed by Schaefer (1991) based on the following characters: ten principal caudal-fin rays; plates at anterior snout margin forming conspicuous spatulate projection; head and body very slender, depressed; dorsal surface of head and trunk with longitudinal ridges; dorsal fin placed posteriorly, well beyond pelvic fin and just anterior to anal-fin origin; adipose fin absent; dermal plates anterior to first dorsal-fin proximal radial expanded, bearing ventromedial keel contacting the dorsal margin of the neural arches of the vertebrae six through twelve; and ridge of bone on the ventral surface of the lateral ethmoid contacting the metapterygoid channel reflected mesially.
Size
Smallest 50mm, largest 67mm, average 61mm, most commonly 63mm. All SL.