The Echinoid Directory

• Test wheel-shaped, depressed above and below.
• Apical disc relatively small (about one-third test diameter, subcircular, with weak indentations interradially and perradially . Plates not bound to corona, always missing.
• Ambulacra straight; plating polygeminate (5-6 elements to a plate), compounded in echinid style, with lowest element the largest, uppermost element also reaching perradius and all intermediate elements demiplates. Pore-pairs undifferentiated; in arcs aborally and ambitally, expanded into short phyllodes adorally.
• A single primary tubercle dominates each compound plate. Only a very few small secondaries and granules at plate corners.
• Interambulacral plates a little wider than tall; dominated by a single primary tubercle, positioned centrally. A few very much smaller secondary tubercles at plate edges on ambital plates. No naked interradial zone present aborally.
• Tubercles imperforate and crenulate.
• Peristome rather large, circular, more or less flush, with large and deep buccal notches with thickened lip and well-developed bar.
• No sphaeridial pits; no basicoronal interambulacral plate.
• Spines unknown.
• Lantern unknown.

Eocene - Oligocene, Europe, Middle East, India, North Africa

Cyphosoma cribrum Agassiz, in Agassiz & Desor, 1846, p. 352, by subsequent designation of Mortensen, 1935, p. 474.

• P. cribrum (Agassiz, in Agassiz & Desor, 1846); Eocene, Lower Oligocene, western Europe
• P. haimei (Desor, 1856); Middle Eocene, Europe

Euechinoidea, Camarodonta (stem group?).

With echinid style plate compounding and deep buccal notches, Porosoma resembles toxopneustid, but differs in having crenulate tubercles and polygeminate rather than trigeminate plating.

Cotteau, G. 1856. Bulletin de la Societe geologique de France, ser. 2, 13, p. 648.