2012-Dec26-HarpocerasThis small impression is about 12mm across.  It is Harpoceras cf exaratum from the lower cliff at Runswick Bay, Yorkshire. The exact OS grid reference is  NZ817155.  The Jurassic shales are full of impressions of ammonites of many types and they are not always easy to identify with precision – hence the ‘cf’ which is an abbreviation of the Latin word Confer, meaning to associate as similar or bring into the same context as. In reality “It looks like this, but we are not certain”.