Praecursor

Cretaceous

Phylum: Echinodermata
Class: Echinoidea
Subclass: Irregularia
Order: Spatangoida
Genus: Micraster

Bilateral symmetry
Five radial ambulacra
Peristome (opening for mouth) and periproct (opening for anus) migrated
Spine attachment scars

Sediment infil
Shell mineral replacement
Disarticulation of spines

Note the streamlined shape for burrowing through the sediment, and how the ambulacra, which bear the tube feet, do not continue onto the underside of the animal where the mouth is.
This is because the tube feet are used for respiration but not for movement, for which small spines are used.