The Echinoid Directory

1a. Ambulacral pores arranged into three discrete columns on oral and aboral surfaces.

1b. Ambulacral pore pairs forming a single irregular column on oral surface; in some forming three rows adapically.

2a.Ambulacra with outer accessory plate large and in contact with adradial suture. Ambulacral zones almost as wide as the interambulacral zones at the ambitus.

2b. Ambulacra with both accessory plates small and occluded in most triads. Ambulacral zones narrower than interambulacral zones at the ambitus.

3a. Buccal notches and gills absent. Primary ambulacral element usually undivided.

Tromikosoma

3b. Buccal notches and gills present. Primary ambulacral element in each triad always divided so as to create an inner column of imperforate perradial plates.

4a. Membranous gaps between interambulacral plates.

4b. No such membranous gaps; interambulacral plates fully abutting.

6a.Large primary tubercles developed orally in interambulacra along adradial margin and interradially, and also present in ambulacral zones.

6b. No prominent marginal row of primary interambulacral tubercles adorally. Primary tubercles absent from ambulacra and perradial zone of interambulacra. Peristomial ambulacral plates uniserial.

7a. Ambulacral plates on peristome arranged quadriserially in each zone. Genital pores open in a membranous gap, and do not pierce the genital plates.

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7b. Ambulacral plates on peristome arranged biserially in each zone. Gonopores open within the genital plates.

8a. Ophicephalous pedicellariae present. Membranous gaps between interambulacral plates only obvious on the oral surface.

8b. Ophicephalous pedicellariae absent. Membranous gaps in interambulacral zones present on both oral and apical surfaces.