The Echinoid Directory

Stem group Neognathostomata with:

• apical system compact and tetrabasal with four gonopores;
• petals developed, open distally; ambulacral plates beyond petals double pored;
• phyllodes well developed; no buccal pores;
• periproct on aboral surface separated from apical disc by interambulacral plates;
• bourrelets: basicoronal plate short, no distinct zone of perioral spines and tubercles;
• no naked granular zone in interambulacrum 5;

Middle Jurassic (Bajocian) to Upper Cretaceous (Cenomanian).

The Nucleolitidae Agassiz & Desor, 1847 as used by Kier (1962) is a plesiomorphic grade in the stem group Neognathstomata, and is here very much restricted in concept. Petalobrissus is identical in most respects, but differs by having single pores beneath the petals. Apatopygidae are more derived in having both single pores beneath the petals and pyrinoid plates in their phyllodes.  The most primitive representative of this group is Pronucleolites.

Agassiz, L. & Desor, P. J. E. 1846-1847. Catalogue raisonné des familles, des genres, et des espèces de la classe des échinodermes. Annales des Sciences Naturelles, Troisième Série, Zoologie : 6 (1846): 305-374, pls. 15-16; 7 (1847): 129-168; 8 (1847): 5-35, 355-380.

Kier, P. M. 1962. Revision of the cassiduloid echinoids. Smithsonian Miscellaneous Collection 144(3), 1-262.