The test (shell)

The urchin’s “shell” is a rigid test of interlocking calcite plates arranged in 20 vertical columns: 10 ambulacral columns (with pore pairs for tube feet) alternating with 10 interambulacral columns (bearing larger tubercles where spines attach).

On a complete fossil you’ll see a central mouth opening (peristome) and an anal opening (periproct), plus a tiny apical system on the top with gonopores and the madreporite.

Spines & jaws

Spines articulate on primary tubercles like ball‑and‑socket joints; they often detach and fossilize separately. Inside the mouth, many regular urchins have a five‑part jaw called Aristotle’s lantern—a favorite microfossil when preserved.

Regular urchins (the classic spiky ball)

• Symmetry: strong five‑fold (pentaradial).
• Shape: globular to low‑dome.
• Mouth & anus: mouth centered on bottom; anus centered on top.
• Lifestyle: grazers on hard or coarse substrates; long robust spines.
• Age notes: common from the Paleozoic onward.

Irregular urchins (hearts, biscuits & dollars)

• Symmetry: still five‑based, but with a front‑back axis (bilateral).
• Shape: heart‑shaped (spatangoids), biscuit‑ or disk‑like (clypeasteroids).
• Top “flower”: petaloid ambulacra—five petal‑like areas of tube‑foot pores.
• Mouth & anus: mouth shifts forward; anus moves to the rear or edge.
• Age notes: diversify in the Jurassic–Cretaceous; sand dollars flourish in the Cenozoic.

Original calcite tests

In limestones and chalks, tests can survive with fine detail: tubercles, pores, plate sutures. Some retain delicate ornament—be gentle; it’s surprisingly crisp but not indestructible.

Molds & casts

Where the test dissolved, sediment can preserve external molds (negative impressions) and internal molds (the infill shape). These often show the petaloid pattern as raised relief.

Replacements & coatings

Groundwater may silicify tests or coat them with pyrite in anoxic settings. Spines are commonly found as isolated fossils scattered in the same layers.

Field ID checklist

• Five‑fold logic: star‑like patterns, five petaloid “flower” on top (irregulars), five rows of pore pairs per petal.
• Tubercles: neat arrays of bumps (spine sockets) on plates.
• Ambitus: the “equator” of the test—often the thickest band of plates.
• Apical system: a small rosette on the top center (regulars); offset in irregulars.
• Spines: cigar‑ to needle‑shaped, with subtle ornament; often found loose.

Palette & textures

• Chalky white in Cretaceous chalks.
• Grey/tan in marls and limestones.
• Iron‑stained golden browns along sutures.
• Silicified pieces with glassy sheen.

Photo tip: Side‑light at ~30° to make pore rows and tubercles pop; a black card behind thin, translucent tests makes the petaloids “bloom.”

Sand dollars vs. “sea biscuits”

Sand dollars (very flat) often show lunules—slots/holes through the test. Sea biscuits (thicker clypeasteroids) lack big slots and look like puffed coins with strong petaloids.

Crinoid bits

Crinoid stems are stacks of discs with a central hole; not a single fused shell with pores and tubercles. Different echinoderm, different vibe.

Blastoids & brachiopods

Blastoids have five petal‑like fields but an overall bud shape and fine plate ornament; brachiopods are two‑valved shells with bilateral symmetry and growth lines, not five‑part flowers.

Concretions

Calcareous concretions can be spherical/ovoid but lack pore rows, plate sutures, petaloids. If you can’t find the five, it’s probably “just a rock.”

Modern shells

Bleached modern tests are ultra‑light and fragile with still‑sharp spines; fossil tests typically show mineral infill, weight, and patina.

Quick checklist

• Five‑fold pattern somewhere? ✓
• Paired pores in bands? ✓
• Rows of tubercles, not growth lines? ✓

Cretaceous chalk classics

Micraster and Echinocorys are famous from the chalk cliffs of northwestern Europe. Tests can be creamy white with superb petaloids and delicate sutures.

Cenozoic sands & limestones

Sea biscuits & sand dollars are abundant in Eocene–Miocene deposits across North Africa, the Mediterranean, parts of the USA (Gulf & Atlantic Coastal Plain), Madagascar, and Australia—great for beginners and display cabinets.

Paleozoic regulars

Early echinoids appear by the Ordovician, becoming more diverse through the Mesozoic. Paleozoic pieces are often rarer and more delicate.

Loose spines everywhere

Layers rich in cidarid spines (large, clubby spines) are common in Jurassic–Cretaceous limestones. They polish beautifully and teach plate‑and‑spine anatomy.

In the field

• Carry small boxes or foam—tests crush easily in a backpack.
• Note orientation and layer: petaloid side up or down? Nearby spines? Context helps ID.
• Bag loose spines separately (future you will thank present you).

Cleaning

• No acids on calcite tests. Use water, a soft brush, and wooden picks.
• For chalky pieces, apply a reversible consolidant (e.g., a light acrylic solution) sparingly.
• Compressed air bulbs remove dust from pore rows without abrasion.

Mounting & display

• Support the ambitus (equator) with a ring of foam or museum putty.
• Side‑light at ~30° highlights petaloids; a simple black card makes patterns crisp.
• Label with age + formation + locality—half the beauty is the backstory.

Find the five

Rotate a test and trace the five ambulacra. On irregulars, the petaloid “flower” should show five petals—with pore pairs running along each “leaf.”

Spine socket safari

Under a 10× loupe, examine tubercles. Many show a smooth boss and a crenulated ring—perfect for a ball‑and‑socket spine joint.