The present monograph is divided into three parts. The first one is dedicated to the lithostratigraphy of the Lias and Dogger in the Tunisian "Dorsale" where we have recognized twelve formations, four of which have not been described before. These lithological units are well documented in several stratigraphic sections dated by abundant ammonites. Several of these sections are strongly condensed. The second part of this work is dedicated to the systematic description of the Lower and Middle Liassic ammonites. Ninety two Saxons including 5 new genera and 17 new species are described: Baltzerites gen. n., Gorgheiceras tuberculatum gen. and sp. n., G. costotuberculatum sp. n., G. costotuberculatum subcarinatum subsp. n., G. planatum sp. n., Galaticeras subtriangulare sp. n., Castanyiceras parvulum gen. and sp. n., Lytoceras galatiforme sp. n., Protocymbites (?) azzouzi sp. n., Protocymbites dennyiforme sp. n., Cheltonia oustense sp. n., Parasteroceras peyssonneli sp. n., Gleviceras sp. n., Foetterleiceras gen. n., Zamaiceras mangoldi gen. and sp. n., Gemmellaroceras subtilicostatum sp. n., Tropidoceras stahliforme sp. n. and Fuciniceras sp. n. Part three of this monograph is attributed to the discussion of the correlation between Tethyan and NW-European ammonite zones. The following zones have been recognized in the tunisian Lias: Semicostatum (?), Turneri-Obtusum, Obtusum, Rothpletzi-Hierlatzicum (="Raricostatum" auct.), Aenigmaticum and Demonense. These zones belong to the stratigraphic interval from the Lower Sinemurian to the Middle Carixian.