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.

depressus Lestre

Jurassic

Phylum: Echinodermata
Class: Echinoidea
Subclass: Irregularia
Order: Holectypoida
Genus: Holectypus

Bilateral symmetry
Five radial ambulacra
Spine attachment scars (very small)
Peristome and anus out of line

Shell mineral replacement
Sediment infil
Disarticulation of spines

This is a primitive irregular echinoid which would have had a functional jaw apparatus (not preserved) held within the peristome. The anus has migrated and the symmetry is bilateral, so this echinoid is clearly identifiable as irregular.