New microconchiate Hybonoticeras from Mexico

A diversified and previously unknown record of microconchiate Hybonoticeras is analysed on the basis of the material collected in sections sampled bed -by-bed in the Mexican Altiplano. Five new species are described and compar ed to species traditionally recorded from the uppermost Kimmeridgian and th e lowermost Tithonian in Europe. A general trend to smoothing in Mexican an d European species during the earliest Tithonian is outlined.