Ptychagnostus is an extinct genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of blind trilobite

Trilobite

Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

 that lived during the Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 period, part of the order Agnostida

Agnostida

Agnostida is an order of arthropod which first developed near the end of the Early Cambrian period and thrived during the Middle Cambrian. They are present in the lower Cambrian fossil record along with trilobites from the Redlichiida, Corynexochida, and Ptychopariida orders...

. Ptychagnostidae generally do not exceed ten millimetres in length. Their remains are sometimes found in the otherwise empty tubes of the polychaete worm Selkirkia

Selkirkia

Selkirkia is a genus of predatory priapulid worm known from the Middle Cambrian Burgess Shale. In the Burgess Shale, 20% of the tapering, organic-walled tubes are preserved with the worm inside them, whereas the other 80% are empty...

. It probably lived in the water column. Its major characteristics are lack of eyes, two lobes on the glabella

Glabella

The glabella, in humans, is the space between the eyebrows and above the nose. It is slightly elevated, and joins the two superciliary ridges.-Etymology:The term is derived from the Latin glabellus, meaning smooth, as this area is usually hairless....

, and three lobes on the pygidium

Pygidium

The pygidium is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. It contains the anus and, in females, the ovipositor...

.

Species

• Ptychagnostus aculeatus
• Ptychagnostus akanthodes
• Ptychagnostus atavus
• Ptychagnostus cassis
• Ptychagnostus ciceroides
• Ptychagnostus cuyanus
• Ptychagnostus fissus
• Ptychagnostus germanus
• Ptychagnostus gibbus
• Ptychagnostus hybridus
• Ptychagnostus intermedius
• Ptychagnostus michaeli
• Ptychagnostus praecurrens
• Ptychagnostus punctuosus
• Ptychagnostus seminula