Hildoceras is a genus of ammonite

Ammonitida

The Ammonitida is an order of more highly evolved ammonoid cephalopods from the Jurassic and Cretaceous time periods, commonly with intricate ammonitic sutures....

 form the upper Lower Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 in the ammonitid family Hildoceratidae

Hildoceratidae

The Hildoceratidae is a family of ammonoid cephalopods from the Lower Jurassic , generally with strongly ribbed, involute shells...

 characterized by a narrow discoidal evolute shell, 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 Hyatt after Saint Hilda in 1876, and has been found in Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 and Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

.