Article: Variation and ontogeny of some Oxford Clay ammonites: Distichoceras bicostatum (Stahl) and Horioceras baugieri (d'Orbigny), from England

Variational and ontogenetic studies of Distichoceras bicostatum (Stahl) and Horioceras baugieri (d'Orbigny) have shown identity in their early stages. Variation in protoconch size is consistent and small, as is the diameter of the nepionic construction. Divergence in shell form occurs only at the onset of maturity, which in H. baugieri begins at about 8-10 mm. and in D. bicostatum at about 30-35 mm. It is concluded that the two 'species' are a sexually dimorphic pair. The name D. bicostatum has priority.