The Echinoid Directory

Tetrapygus Agassiz, in Agassiz & Desor 1846, p. 354

[= Echinocidaris Duncan, 1889, p. 94, non Desmoulins (objective)]

Diagnostic Features
  • Test flattened below, gently domed above.
  • Apical disc small; hemicyclic; plates firmly bound to corona. Ocular pores double. Periproct oblique, covered by four large plates forming an anal valve. No tubercles, but with distinct epistromal ornament. Oculars usually insert, but can have posterior oculars exsert.
  • Ambulacra more or less straight; plating polygeminate at ambitus and adapically; pore-pairs offset in steep arcs on each plate so as to define a broad adradial band above; with suckerless tube-feet, the pore-pairs elongate and conjugate. Pore-pairs expanded to form strong phyllodes adorally, with rounded pore-pairs and strong, suckered tube feet.
  • Plate compounding arbaciid-style with three or four demiplates plus a large element forming each compound plate. There is a single large primary tubercle on each compound plate.
  • Interambulacral plates wide with up to 5 subequal primary tubercles forming a horizontal row on each plate at the ambitus (fewer above and below). On the upper surface there is often a second adapical row of smaller tubercles above the row of primaries. Outer series of tubercles continuing to apex. No interradial naked zone adapically.
  • Primary tubercles imperforate and non-crenulate, with relatively large mamelon and poorly defined platform. On oral surface forming en-chevron pattern.
  • Peristome subpentagonal and about half test diameter, with shallow buccal notches and poorly developed tags.
  • Peristomial membrane with many small plates.
  • Perignathic girdle with rather tall and slender auricles, generally fused perradially.
  • Primary spines moderately long (up to half test diameter); adapical spines distally pointed, without cortex; oral and ambital spines ending in a small cap of glassy stereom (cortex).
  • A single sphaeridium situated perradially close to the peristome in a pit.
  • Recent, Peru & Chile.
Name gender masculine
Echinus niger Molina, 1782, p. 175, by monotypy.
Species Included
  • Only the type species.
Classification and/or Status

Euechinoidea, Echinacea, Arbacioida, Arbaciidae.



Distinguished from Arbacia by its polygeminate ambulacral plating, with four elements composing a compound plate, not three.

Although Agassiz (1841, p. vii) is often quoted as the author this is a nomen nudum as the genus name appears without description or reference to a species.  The first valid use of the name thus appears in Agassiz & Desor (1846).

Agassiz, L. 1841. Observations sur les progress récens de l’histoire naturelle des echinoderms. Monographie d’Échinodermes vivans et fossiles. Seconde Monographie, première section, 20 pp. Petitpierre, Neuchâtel.

Agassiz, L. & Desor, E. 1846. Catalogue raisonnee des echinodermes. Ann. Sci. Nat. 6, 305-374.

Mortensen, T. 1935. A monograph of the Echinoidea II. Bothriocidaroida, Melonechinoida, Lepidocentroida and Stirodonta. C. A. Reitzel, Copenhagen.

Schultz, H. 2005. Sea Urchins. H. & P. Schultz Partner Scientific Publications. Hemdingen.