What is "sphaeragnostus"

Sphaeragnostus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It can be recognized by having two thorax segments, a totally effaced headshield (or cephalon), while the tailshield (or pygidium) although effaced, has a clear furrow parallel to its border, and a short, convex, subcircular axis. It lived during the Ordovician (upper Tremadoc to Ashgill).