The Trilobite Phacops rana is the state fossil of Pennsylvania. I can remember the campaign that my club at the time assisted with to get this common Trilobite nominated. It is an ubiquitous fossil in Devonian rocks and no less so in the Mahantango. Below are some pictures of specimens I've found from the Mahantango.
A partially enrolled specimen
This is the best enrolled Phacops that I ever found from the Mahantango.
How many Trilobites can you find in this picture?
I've heard that the genera name Phacops has been superseded by Eldredgops but I'm finding it hard to locate information to confirm that fact. It seems to be a 50/50 battle among sellers with most references to Eldredgops being assigned to Trilobites from the Silica Shale.
Speaking of which, here is an enrolled specimen in situ from the Silica Shale in Sylvania, Ohio.
I checked the Paleodb.org website and found plenty of info for the genera Phacops, but when I put Eldredgeops in the search it only came up with a family name but no references other than this : J. J. Sepkoski. 2002. A compendium of fossil marine animal genera. Bulletins of American Paleontology 363:1-560 [J. Alroy/J. Alroy/M. Carrano]. It seems that Sepkoski has been busy writing papers as I often see his name associated with Devonian period fossils. However the one reference to Eldredgeops isn't even a paper which describes the reasoning, it's just a list of genera.
This site (http://www.fossilmuseum.net/Fossil_Galleries/TrilobitesOhio/crassituberculata/phacops.htm) even gives you a reason for why "some prefer" Elderdgeops.