Sismondia occitana Echinnoid
Calcaire de St. Estephe of France
Eocene Era (56 to 33.9 million years ago)
This nice little echinoid is from the Calcaire de St. Estephe of France. The Sismondia occitana has a diameter of 3/4". Diagnostic Features: Small, ovate to subpentagonal in outline with rounded ambitus. Oral surface sunken towards peristome. Apical disc central with four gonopores. Hydropores in a groove. Petals open distally; weakly bowed. Ambulacral zones much wider than interambulacral zones at ambitus. Peristome central; small and circular. No food grooves. Periproct opening on oral surface towards posterior margin; at contact between the first and second paired post-basicoronal plates. Basicoronal interambulacral plates projecting slightly beyond first paired ambulacral plates. Paired ambulacral plates pointed and projecting perradially. Second paired ambulacral plates not enlarged. Tubercles on oral surface small and dense with sunken areoles. Ten radial partitions with lateral outgrowths forming incipient concentric laminae. Differs from other members of the Laganidae in having relatively simple internal butressing. In Sismondia there are 10 radial struts which have three to five short lateral branches that form incipient cicumferential partitions. In other genera the circumferential partitions are well developed. Also the basicoronal ambulacral plates are less pointed perradially than in other taxa.