The Echinoid Directory

• Test plating moderately thin; with imbricate sutures but no membranous gaps.
• Apical disc hemicyclic with ocular plates II and III insert in juveniles but monocyclic in adults. Gonopores passing through undivided genital plates.
• Ambulacral about one-third of interambulacral width at ambitus.
• Ambulacral plating trigeminate and pseudocompound; with large middle element and two small demiplates, both in contact with the adradial suture. Pore-pairs in arcs of three adapically and at the ambitus, forming broad phyllodes close to the mouth.
• Single primary tubercle on every compound plate. These tubercles variable in size, with larger tubercle on every second or third plate adorally. Adorally the trigeminate elements become markedly variable in size, with every third much larger.
• Interambulacral zone composed of wide, weakly geniculate plates. One adradial primary tubercle on adapical plates and two or three subequal tubercles on oral surface.
• Primary tubercles perforate and non-crenulate.
• Peristome with broad, rounded buccal notches.
• Spines slender, smooth and with spongy fill; without hyaline hoof.
• Ambulacral plates quadriserial over outer part of peristome.

Middle Jurassic (Upper Bajocian), Switzerland

Pelanodiadema oolithicum Hess, 1972, p. 42 by original designation

• Only the type species

Euechinoidea; Echinothurioida, Pelanechinidae

Monotypic. Subjective junior synonym of Pelanechinus Keeping, 1878

Hess (1972, 1973) gives a full and well illustrated description of the type species. Although Hess (1972) made careful comparison with Pelanechinus, he based this on inaccurate early descriptions. In fact Pelanechinus appears to differ in no significant details from Pelanodiadema.

Hess, H. 1972. Eine Echinodermen-Fauna aus dem mittleren Dogger des Aargauer Jura. Schweizerische Palaontologische Abhandlungen 92, 1-87, pls 1-23.

Hess, H. 1973. Neue Echinodermenfunde aus dem mittleren Dogger des Aargauer Juras. Eclogae geologica Helvetica 66, 625-656.