Wikipedia
Hildoceras
Hildoceras is a genus of ammonite form the Jurassic in the family Hildoceratidae. The shells are characterized by a narrow discoidal evolute shape, keeled venter, concave ribs along the outer flanks, and a shallow spiral goove running along smooth inner flanks. Whorls slightly overlap, cross sections are compressed. The ventral keel is bordered on either side by a shallow groove.
The genus was named by Alpheus Hyatt after Saint Hilda in 1876, and fossils have been found in Europe and Japan.