The recent discovery of lower Bathonian (upper middle Jurassic) ammonites at several places in the Bugey region of the southern Jura mountains of eastern France resolves the problem of the relationship of the Bajocian to the Bathonian in this key region. Fauna of upper Bajocian (lower middle Jurassic) and middle and upper Bathonian had long been known but lower Bathonian was unknown. One bed is particularly rich in a varied fauna of ammonites, pelecypods, gastropods, and echinoderms in a remarkable state of preservation. The faunal association shows the coexistence of the forms of the Zigzag beds of England and the Wuerttembergicus beds of Germany, thus confirming the homology of these two horizons. The geographic distribution of type genera of the lower Bathonian in western Europe permits the identification of two faunal domains overlapping in Lorraine and the northern Jura mountains as well as the Bugey region. The genus Zigzagiceras extends from southern England through Switzerland, the Jura mountains, and the Basses-Alpes to Sicily, and also has been found in Portugal. The western limit of the species Parkinsonia (Oraniceras) wuerttembergica occurs in Germany, Lorraine, the Jura mountains, and Sicily. It appears that the genera Morphoceras and Ebrayiceras may have an even greater distribution.