Noun
The class Echinodermata which includes the sea urchins. They have a calcareous, usually more or less spheroidal or disk-shaped, composed of many united plates, and covered with movable spines. See Spatangoid, Clypeastroid.
Source: Webster's dictionaryThe most glamorous thing to be said for these members of the Echinoidea family appears to be that they were named in honor of the Middle English nomenclature for hedgehogs. Source: Internet
Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint). Source: Internet