Legend has it that St. Hilda, founding abbess of Whitby abbey, turned a plague of snakes to stone and threw them off the cliff, thus creating the ammonites we can find there today.
This legend was immortalized by Alpheus Hyatt in 1876 by naming the ammonite genus I´d like to present to you next after her: Hildoceras.

In Victorian times, snake heads were often carved on ammonites to capitalize on this legend – today there seems to be a renaissance of this art going on, and the quality
you´ll find on places like ebay is usually quite good, although these are mostly Dactylioceras ammonites.

But back to palaeontological reality :

In Yorkshire, there are 3 species of Hildoceras (I´m following HOWARTH´s “The Ammonite Family Hildoceratidae in the Lower Jurassic of Britain” here)  :

Hildoceras bifrons is the lead ammonite for the bifrons zone of the lower Toarcian. It has a strong spiral groove, strong ribbing on the external half of the side,

and a keel with strong side furrows.

Hildoceras semipolitum has a higher whorl section than H. bifrons, both spiral groove and ribbing are finer. The outer whorls overlap the inner whorls to a degree that

the ribs on the inner whorls are no longer or just about visible. As H. semipolitum is considered a descendant of H. bifrons, there are many intermediates.

Hildoceras lusitanicum (also known under the synonym Hildoceras sublevisoni) is very similar in ribbing to H. bifrons,
the main difference is the missing spiral groove on the side of the whorl.

AndyS