Catagenesis in a series of Productus from the Liddar Valley in Kashmir

Four different species ofProductus frequently found, in the Lower and Middle Carboniferous of Kashmir show a lineage more or less in the stratigraphical succession. It starts fromProductus cora d'Orb. (the primitive member of this series) and exhibits a retrogressive accentuation of the twisting of the umbo which almost disappeares in the youngest specimens. The spiral angle varies in the four stages from 70° to 30°. The lineage is regarded as a true genetical one.