Hemicidaris intermedia Sea Urchin, from Ardennes, France (REF:HEMSU1)

Hemicidaris is an extinct genus of sea urchin that lived from the Middle Jurassic to the Early Cretaceous.

Sea Urchins (also known as Echinoids) have been found as fossils dating far back as the Middle Ordovician period 465 Million Years Ago and there are still about 950 different species still in existence today, inhabiting a wide range of depth zones and climates throughout the worlds oceans.

The name urchin comes from a Middle English word meaning Hedgehog because of its spiny appearance, but, not all Sea Urchins have long spines in fact some have a hair-like structure which covers the body. 

Fossils of urchins with spines can be found, while others will be just the shell (or test) with small bumps across the test called tubercles where the spines would have attached. Those that are slightly smoother or with smaller tubercles on the test would have been the urchins with the hair-like structure covering the shell.

Urchins are placed into two categories; regular and irregular. This is simply explains what species they are and their shape rather than rarity. Regular (spiny urchins, generally found on rocky substrates) have no front or back end and can move in any direction. Irregular urchins have a definite front and back and can only move in a particular direction (Sand Dollar urchins for example).

Sea Urchins are apart of a family of marine life called Echinoderms. The Echinoderms include the Echinoids (Sea Urchins), Holothuroids (Sea Cucumber), Asteroids (Starfish ), Crinoids (Sea Lilies), as well as the extinct Blastoids, Cycstoids, and Carpoids.