Wikipedia
Asthenoceras
Asthenoceras is a dwarfish, Middle Jurassic (Lower Bajocian) ammonoid with an evolute, smooth, compressed, discoidal, strongly keeled shell. Asthenoceras belongs to the Hildoceratidae (subfamily Grammoceratinae) and may be a subgenus of the Lower Jurassic (Upper Toarcian) Grammoceras.