Sea Urchins
Class Echinoidea (Leske, 1778)

 Sea urchins are members of a much larger group of marine animals called echinoderms. These include sea stars, brittle stars, crinoids and sea cucumbers. The fossil record shows divergence from sea stars nearly 450 million years ago in the late Ordovician period. There are about 7000 extinct species of sea urchins and about 900 living species. The sea urchin shell or skeleton is called a test, which protects the soft internal organs, and is covered with spines arranged in five broad areas that are separated by narrow unprotected areas. Fragments of the spins and tests can be common place in some locations but finding complete ones can be a challenge.

Modern Sea Urchin¹
The sea urchin shell or skeleton is called a test, which protects the
soft internal organs, and is covered with spines arranged in five broad
areas that are separated by narrow unprotected areas.

Catopygus (Oolopygus) williamsi Clark
Identification by Ralph Johnson curator of the MAPS collection
(Monmouth Amateur Paleontologist Society)

Navesink Formation
Cretaceous
Monmouth County, New Jersey

Salenia sp.
Bottom right Apical Disk
Paleocene, Burlington County, New Jersey

Catopygus sp.
Paleocene, Burlington County, New Jersey
 

Hemiaster sp.
Paleocene, Burlington County, New Jersey

An interesting specimen with the chewing organ preserved.
The entire chewing organ is known as Aristotle's lantern. So called from a
 reference by Aristotle to a sea urchin resembling certain hand lanterns
of his time.
Acrosalenia hemicidaroides Middle Jurassic, NE France

¹ Photo in public domain