Dactylioceras semicelatum Ammonite fossil with box and stand - Whitby, North Yorkshire Jurassic Coast Yorkshire FossilsAmmonite fossil found near Whitby, North Yorkshire on the Jurassic coast Generally oval whorl section although extremely variable species (from compressed flattened to very depressed thick forms). Straight (or gently convex), relatively fine, well defined, dense ribbing. Ribs run straight or slightly bent forward across the top (venter). Maximum size ~11cm, average size ~7cm. Actual Ammonite size: 5. 5cm Age (lithology): Upper Lias; Toarcian;