Agnostus was a blind, simply shaped trilobite reaching the size of a little fingernail. The cephalon, or head end, lacked the usual grooves or sutures that separate its central glabella from the side-cheeks. The pygidium, or tail, was rounded and smooth. In addition, Agnostus had only two segments in its thorax. This was the result of a pattern of thoracic segment reduction that began in the Early Cambrian and reached its peak at the end of that period. Agnostus probably lived on the deep ocean floor, using its short antennae to sweep up food particles from the sediment.