The Echinoid Directory

Key to the major stirodont clades


Go to 2 1a. Apical disc large; one or more periproctal plates tesselated around interior into ring creating an angular border.	1b. Apical disc small with than peristome; hemicyclic or dicyclic always without periproctal plates tesselated into the disc. Go to 3

2a. Apical disc firmly bound to corona; ambulacral tubercles always much smaller than corresponding interambulacral tubercles. Salenioida	2b. Apical disc not bound to corona and usually missing; ambulacral and corresponding interambulacral tubercles approximately similar in size. Phymosomatoida

3a. Primary tubercles perforate . Go to 4	3b. Primary tubercles imperforate and non-crenulate. Go to 6

4a. Interambulacral and ambulacral plates dominated by single large tubercle. Go to 5	4b. Interambulacral and ambulacral tubercles small and multiple [perforate and crenulate nature of tubercles often rudimentary at best]. Pedinopsids

5a. Primary interambulacral tubercles much larger than ambulacral tubercles on aboral surface. Hemicidaridae	5b. Ambulacral zones not particularly narrow adapically; compound to apex . Pseudodiadematidae

6a. Primary tubercles crenulate. Glyptocidaris	6b. Primary tubercles non-crenulate. Go to 7

7a. Ambulacral plate compounding diadematid with all elements reaching the perradius. Stomechinidae	7b. Ambulacral plate compounding arbaciid or stomopneustid with small occluded plate(s) bearing only pore-pairs. Go to 8

8a. Ambulacral plating trigeminate (rarely quadrigeminate). Arbacioida	8b. Ambulacral plating with more than 5 elements to a compound plate. Stomopneustidae