This is a very hard-to-find trilobite. It is a Pseudogygites latimarginatus from Ontario, Canada. You may have seen a few Pseudogygites from the black shale Whitby Formation, but how many have you seen from the Cobourg Formation? Only the top few feet of this very thick formation have Pseudos present.
Pseudogygites is similar to Isotelus, but not the same in a few obvious ways - Pseudos have ribs on the tail, eyes with large facets, and they do not possess a median suture crossing the doublure that the other asaphid trilobites have!
This specimen is very large - over three inches long! It was found well over 10 years ago when these few layers in the quarry were being worked. This is a very large example of a rare Pseudogygites latimarginatus from the Cobourg Formation.