Stephanoceras Ammonite, from Sherborne Dorset, England, UK (REF:STEPAM1)

Stephanoceras (meaning crown horn) is an extinct genus of ammonite which lived during the Middle Jurassic.

  

Stephanoceras has an evolute shell, as characteristic of the family, with well-developed ribbing and tubercles. The shell is coiled so whorls barely touch in most, but some have notable overlap. Primary ribs emerge from the umbilical seam, the line marking the inner edge of the particular exposed whorl, and divide on the flanks, usually in two, occasionally in three, forming secondary ribs that cross the outer rim of the shell, known as the venter, uninterrupted. Turbercles, elevated projections, form at the ends of the primary ribs where they bifurcate or sometimes trifurcate.

Stephanoceras grew to be fairly large with a shell diameter as much as 27cm and width as much as 6cm across the outer whorl.