The Echinoid Directory

• Test small, composed of thin imbricate plates. In life subglobular; taller than wide.
• Apical disc small; 25% of test diameter at most. Five small ocular plates but only a single enlarged genital plate with hydropores. Central zone filled with small periproctal plates.
• Ambulacra more or less straight and narrow; biserial. Plating simple with rounded pore-pair on each plate.
• Pore-pairs lie close to perradial suture with a single large tubercle on the adradial side of each. Broad adradial flanges extend beneath adjacent interambulacral zones.
• On the inner surface of ambulacral plates there are strongly developed flanges projecting perradially. The radial water vessel is thus fully enclosed within the ambulacral plates.
• Interambulacral zones wide; composed of four semi-regularly arranged columns of rhomboidal plates; largest adradially. Plates imbricate towards the interradius.
• All interambulacral plates bear a small imperforate and non-crenulate granule at one edge but are otherwise devoid of granulation.
• Peristome small, covered in ambulacral plates with spines.
• Lantern present; small, with grooved tooth.
• Spines of two sizes present only on ambulacral plates; relatively short and straight, without cortex.

Lower Silurian (Llandovery), Scotland.

Aptilechinus calidonensis Kier, 1973, p. 654, by original designation. Holotype, Royal Scottish Museum, Edinburgh 1897.32.537B.

• Only the type species.

Stem group Echinoidea; Lepidocentridae.

Monotypic.

This taxon has rhomboidal interambulacral plates with a single offset tubercle that are characteristic of lepidocentrids. It differs from all other genera in this family by the small number of interambulacral columns that are developed: 4 as opposed to at least 7 or 8 in other taxa.

Kier, P. M. 1973. A new Silurian echinoid from Scotland. Palaeontology 16, 651-663.