The Echinoid Directory

Periarchus Conrad, 1866, p. 21

Diagnostic Features	Test with sharp margin; circular in outline, distinctly conical in profile. 5 gonopores. Petals well developed: straight and closing a little distally. Ambulacral and interambulacral zones about the same width at ambitus. All interambulacrum continuous on oral surface but basicoronals only just in contact with first paired interambulacral plates. Basicoronal circlet strongly pentastellate, with interambulacral plates projecting. Periproct oral, close to the peristome; opening bounded by first paired interambulacral plates. Food grooves comprising short primary trunk which branches after second pair of ambulacral plates; without finer branches towards the periphery.
Distribution	Late Eocene of South East USA.
Name gender	masculine
Type	Sismondia alta Conrad, 1865, p. 74, by original designation.
Species Included	P. alta (Conrad, 1868); Late Eocene, USA. P. lyelli (Conrad, 1868); Late Eocene, USA.
Classification and/or Status	Clypeasteroida; Scutellina; Scutelliformes; Protoscutellidae. Monophyletic.
Remarks	The position of the periproct close to the peristome and within the first post-basicoronal pair of interambulacral plates distinguishes this taxon from Protoscutella, while the thin margin distinguishes it from Mortonella. Durham, J. W. (1955) Classification of clypeasteroid echinoids. California University Publications in Geological Sciences 31, 73-198. Kier, P. M. 1980. The echinoids of the Middle Eocene Warley Hill Formation, Santee Limestone, and Castle Hayne Limestone of North and South Carolina. Smithsonian Contributions to Paleobiology 39, 1-102, pls 1-22.