Today I have a rather odd bug to show you. It's a partial cephalon from a Trilobite called Bristolia insolens. It comes from the Latham shale of the Marble Mountains in California. B. insolens is odd in that it has a pair of genal spines that are attached to the front of the cephalon rather than extending from the sides or rear corners.
This particular species is well known and specific to a time period in the Cambrian and is used to designate a faunal zone. That means that if you are finding specimens of B. insolens in the rock then you should be able to find a very specific set of other fauna associated with it as well and it has a very specific age. In this case, B. insolens is isolated to the base of the Wuliuan stage of the Miaolingian series within the Cambrian period (509 mya).
A complete specimen of this species of trilobite can be seen at the Western Trilobites webpage here.
I used the Wikipedia page on the genus Bristolia to fill out some of the details of my post.