Since it was an unknown trilobite to me I consulted the BEST trilobite fossil site on the internet Sam Gon III's http://www.trilobites.info/. It appears to me after looking at his drawings and images on the site, this speciment might be an Elrathia kingii. Specifically, the site lists it as Order: Ptychopariida, Suborder: Ptychopariina, Superfamily: Ptychoparioides, Family: Ptychopariidae. The site has a picture gallery with references to pictures on another site http://www.trilobite.com/ which sells Elrathia kingii trilobites that lived during the Middle Cambian in Utah. Also I believe it or another site noted that this trilobite is the most common one sold today that is intact.
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Part of the pygidium is missing. I count 12 thorax segments.
Bottom of plate that contains the trilobite. Rock is almost black to very dark gray in color.