Ptychagnostus is an extinct genus of blind trilobite that lived during the Cambrian period, part of the order Agnostida. Ptychagnostidae generally do not exceed ten millimetres in length. Their remains are sometimes found in the otherwise empty tubes of the polychaete worm Selkirkia. It probably lived in the water column. Its major characteristics are lack of eyes, two lobes on the glabella, and three lobes on the pygidium.