Ordovician to Permian

Phylum: Cnidaria
Class: Anthozoa
Order: Rugosa

Septa
Growth lines
'Horn' shape

Manmade cast

Crinoids: Ordovician to recent

Phylum: Echinodermata
Class: Crinoidea

Ossicles (fused)

Internal mould of stereome by haematite, slightly compressed

Carboniferous

Phylum: Echinodermata
Class: Crinoidea

Ossicles (fused)
Stereome

Mineral replacement
Sediment infill

Encrinus lilliformis

Triassic

Phylum: Echinodermata
Class: Crinoidea
Order: Encrinida
Family: Encrinidae
Genus: Encrinus

Ossicles (fused)
Crown with individual arms
Attachment point for calyx

Cast (manmade)

Jurassic

Phylum: Echinodermata
Class: Crinoidea
Order: Millericrinida
Genus: Apiocrinites

Pentaradial symmetry
Calyx
Attachment point for 'arms' of the animal

Cast (manmade)

Phylum: Echinodermata
Class: Crinoidea

Stem
Calyx
Crown
Ossicles (fused)

Cast (manmade)

Florigemma

Jurassic

Phylum: Echinodermata
Class: Echinoidea
Order: Cidaroida
Genus: Cidaris

'Club' shaped
Thorns

Mineral replacement (calcite)
Disarticulated from body shell

This is a spine from a regular echinoid. You can tell this by the shape and thickness of the spine.
Irregular echinoids live under the sediment in burrows, using their spines for movement, not for protection, and hence the spines are fine and able to be manipulated.
Regular echinoids live epifaunally, and so are a target for predators. These spines are needed to prevent attack of the shell containing the soft parts of the animal, and the 'club' like shape decreases access opportunities for predators. Other regular echinoid spines are long and thin, pointed, offering a more 'offensive defence' than spines such as this one.

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.

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.

Cretaceous

Phylum: Echinodermata
Class: Echinoidea
Subclass: Irregularia
Order: Echinoneoida
Genus: Conulus

Bilateral symmetry
Five radial ambulacra
Interambulacra
Migrated periproct (opening for anus)
Peristome (opening for mouth)

This fossil was collected in Quaternary river gravel. Originally it was preserved in the Upper Chalk (Cretaceous). This was broken up into clasts by erosive processes, one of which was this irregular echinoid. The echinoid clast was carried by a river downstream and deposited in the river gravel unit during the Quaternary.

Note how the anus has migrated, but the peristome is still in the position you would expect from a regular echinoid (not migrated). Notice also how the ambulacra bearing the tube feet continue onto the underside, a feature not seen in more streamlined and derived irregular echinoids.