An almost round, slightly flattened urchin that grows up to 57 mm in diameter (although more typically to 35 mm diameter). It is greenish in colour with distinctive violet tips to the spines. The spines are robust, short and closely packed.
- Sometimes called the purple tipped sea urchin. Older publications may refer to sea urchins as "burrs" (Hancock, 1957). Strongylocentrotus droebachiensis is also known as the green sea urchin. It is possible for Psammechinus miliaris to be confused with Strongylocentrotus droebachiensis or pale specimens of Paracentrotus lividus.
- A regular sea urchin with a somewhat flattened test. The colour varies with habitat (Bull, 939; Lindahl & Runnström, 1929; Comely, 1979). Shallow water or littoral individuals are a deep purplish-brown and show no difference between the colour of the test and spines. Those from deeper water tend to be paler in colour, with a light green test and vivid purple spine tips.
- The tube-feet are arranged in arcs of 3, visible as 3 pairs of pores corresponding with each ambulacral plate on the denuded test.
- A typical species of bouldered sheltered shores, also found sublittorally in shallow water in sheltered or slightly brackish sites such as sea lochs. Common in the circalittoral on exposed shores in Shetland.