Pliensbachian and Toarcian molluscs and brachiopods of Morocco, Iberia, England, and Greenland are analysed in terms of diversity and density. With regard to local variations, Substrate and food supply are thought to be the major factors controlling the predominance of suspension-feeding bivalves in shallow water deposits while the greater physical stability of deeper water favoured the ammonites; high diversity of the gastropods relates to firm substrates. On a regional scale, both bivalve density and diversity tend to increase from the Tethyan to the Boreal redm, while there is a Variable tendency for a northward diversity reduction in other groups. The bivalve data are incompatible with either temperature or salinity control of faunal provinciality. To provide a better account of this, a model is developed which lays emphasis on the physical instability of a boreal inland sea as compared with the Tethys. Some broader implications for endemism in Jurassic faunas are briefly discussed.