Elrathia

Elrathia is a genus of ptychopariid trilobite species that lived during the Middle Cambrian of Utah, and possibly British Columbia. "E. kingii" is one of the most common trilobite fossils in the USA locally found in extremely high concentrations within the Wheeler Formation in the U.S. state of Utah.
"E. kingii" has been considered the most recognizable trilobite. Commercial quarries extract "E. kingii" in prolific numbers, with just one commercial collector estimating 1.5 million specimens extracted in a 20-year career. 
Etymology.
Even though the generic name "Elrathia" was first published in the combination "E. kingii", a species from the House Range, Utah, the name, itself, is derived from Elrath, Cherokee County, Alabama.
Description.
"E. kingii" is a medium-sized trilobite with a smooth sub-ovate carapace that is tapered towards the rear. Thorax is usually 13 segments. Pygidium has four axial rings and a long terminal piece. Posterior margin of the pygidium has a long broad medial notch.
In contrast, "E. marjum" usually has 12 segments, 5 axial rings, lacks a notched posterior margin and possess incipient antero-lateral spines.
The British Columbian species, "E. permulta", is much smaller, averaging about only 20 millimeters, and has up to thoracic 14 segments. Because "E. permulta" lacks several diagnostic features of the genus it may even represent a distinct genus.
Synonyms.
"Elrathia" is variously known as "Elrathina", which is a separate genus sometimes considered to be a synonym of "Ptychoparella." The species "E. kingii" is often erroneously called "E. kingi" (with one "i").